In the old days, defining a sub-genre of music was different than it is today. It wasn't long after the birth of Thrash Metal that Black and Death Metal were also growing from the same roots, in a sense. Sometimes, certain bands belonged to two or more categories, right from the beginning. For example, Slayer was as much a Black Metal band as Venom, yet they were also Thrash. The full transition would come a few years later, which will soon be touched upon.
Early on, the Thrash movement yielded a multitude of high-quality bands like Metallica, Slayer, Overkill, Anthrax, Megadeth, Exodus, Kreator, Sacrifice, Razor, etc. Strong scenes developed in New York, the Bay Area, Canada, Germany and even Brazil. They all managed to produce great albums that would stand the test of time as true genre-defining classics. The very best records to come from this movement, for the most part, emerged in the first few years. This work had a strong impact on those bands who followed, all throughout the Metal scene, regardless of whichever sub-genre they belonged to. Eventually, several bands were “converted” and made the transition. Slayer gave up the Satanic topics and dark atmosphere for a more pure Thrash sound with Reign in Blood. Sepultura, Sodom and Possessed are examples of bands that did the same, with Schizophrenia, Persecution Mania and Beyond the Gates, respectively. Even Kreator cleaned up their sound and lost the raw Black/Death feeling for the more streamlined sound heard on Terrible Certainty.
In time, more bands would follow, seeking to emulate or even out-do those that had inspired them. As record labels saw the success of bands like Metallica and Slayer, they began to sign tons of new groups. Often, these bands really had nothing to contribute. As with most musical movements, you have the pioneers that blaze the trail and then you have the legions of followers whose primary concern is to simply add their own perspective or to just follow in the footsteps of those that went before them. Several solid records were released as a result of this. Even if some of the newer bands weren't doing anything groundbreaking or unique, they were working within the newly defined boundaries of Thrash Metal and making quality music. Sadly, as time went on, many others jumped on the bandwagon with nothing to add but sub-par imitation and ended up contributing to the demise of the music that they valued.
Just as Thrash Metal was reaching the height of its popularity, many of the primary bands had started to lose their creative spark and to lower themselves to the level of their followers. It didn't take long for some of these bands to lose their edge and abandon the principles on which they were founded. After two impressive albums, Metallica released the dumbed-down disappointment known as Master of Puppets. And while the masses ate it up, it represented the end of an era for that band. They chose to stay within their safe zone and make boring music that was more easily accessible. Slayer did the same with Reign in Blood, dropping the more complex song structures and evil atmosphere for something simplistic and easier to digest. And while a band like Testament was a latecomer, not releasing their debut album until 1987, their follow-up was bereft of any real ambition.
The lyrical approach also took a nosedive. Many bands embraced the excesses of the age while others sought to use their music as a tool to preach their political beliefs. This ended up with a lot of bands either becoming too serious or not serious enough. The “party hard” feeling that was conveyed by some bands did a lot to perpetuate the stereotype of the “dumb metalhead” and alienated some listeners that didn't care to hear drunken Neanderthals praising alcohol and other empty substances. This often led to bands writing about goofy topics, and Anthrax can be blamed for a lot of this. The 1987 release of I'm the Man, singlehandedly, undermined nearly everything that they had accomplished and would set a precedent for the hideous and repulsive mixing of Metal with rap.
On the other end of the spectrum, there were bands that were using their lyrics to convey more serious messages but, often, messages that listeners didn't care to hear. Perhaps, it was one of the influences from Punk Rock, but the tendency to go on about corrupt politicians or televangelists was a real turn-off. Take a record like Peace Sells... But Who's Buying?, for example. Ten times out of ten, I prefer to listen to “The Conjuring” or “Devil's Island” over the title track. Some people don't care about politics, or would rather not have to think of such things while they are trying to listen to music. Others might care, but then the songs become quite dated as time goes on. For all the bands that took shots at Jimmy Bakker or the PMRC, how many of today's generation even know what that was all about? The comment on the Cold War by writing a song about nuclear holocaust is a bit more clever than being so obvious about it and making it completely irrelevant a short time later. I don't believe that politics need to be mixed in with Metal, whether it's Thrash or NSBM. However, I'm willing to concede that this may only be a personal preference.
The aesthetics of Thrash Metal also suffered. Cover art went from scenes of violence and horror to drunken band members drinking beer in front of the television. Or, often times, the artwork reflected the political nature of the lyrics. Also, in the early days, it seemed the the Metal look was more defined. In time, the basic Thrash Metal image became the standard jeans, high-top sneakers, band t-shirts and maybe leather or denim jackets. Then came State of Euphoria. Anthrax's whole image changed from guys wearing black jeans and leather jackets, with some rather serious sounding riffs, to being ridiculous clowns that wore shorts that a lot of girls would even be embarrassed to wear in public. I've always maintained that the aesthetics of an album should be a part of the overall presentation. While the bands don't all have to be trying to look evil, holding axes and chains, they should at least appear as serious as the music contained therein. In the case of Anthrax, the aforementioned album isn't really as goofy as the packaging would make it seem, but it makes it more difficult to enjoy the full experience when the band members look like they're on their way to a gay beach party.
In the late 80s and early 90s, Thrash Metal was dying. Most of the early bands were merely shadows of their former selves. Some had fallen into a rut of self-imitation and going through the motions, rehashing old ideas, while others were simplifying their sound and making radio-friendly “hits” to appeal to the mainstream. The process started before the 80s came to a close, but Metallica's Black Album certainly signaled the end, moreso than any other single release. As they completed what they'd begun a few years earlier, selling out and opting for the stadium Rock route, others tried their best to follow in their footsteps. Megadeth, Testament and Anthrax all made weak records that were more accessible and boring, trading the Thrash for mid-paced groove riffs and catchy choruses. A handful of bands opted for a more intense approach, fading back into the underground.
During the years that Thrash was beginning to become stale, just before the decline, Death Metal had filled the vacuum that existed in the underground. The cycle repeated itself, as the classic period of Death Metal coincided with the stagnation and eventual collapse of Thrash. And, of course, once Death Metal had run its course and followed the same pattern that Thrash had, albeit on a smaller level, the Second Wave of Black Metal rose up and carried on the true spirit of the underground.
These days, Thrash Metal is experiencing somewhat of a revival. Some of the old bands have reformed and are doing their best to return to their roots, but it all stinks of modernity. As well, the retro bands that sprang up in the past decade leave a lot to be desired and have done nothing to rekindle the old flame. They may be picking up where many of the original bands left off in the late 80s, but very few are even striving to match the achievements of Thrash Metal's classic era.
Thrash Metal was born of the underground and it was meant to remain there. Like any movement that pokes its head up into the mainstream world long enough to be noticed, it became just another victim of the music industry.