Now that thrash is making a semi come back all of a sudden Metallica is a thrash band again.
I will give my respects to bands like Slayer, Testament, Exodus, Kreator, etc who have always been Thrash even at a time when that style of music wasn't 'popular' anymore.
I agree with most your post above, yet wanted to highlight the above. Thrash will never be mainstream, but I am not sure what's mainstream anymore (I don't listen to modern "metal" records that are not heavily thrash or classic metal influenced, for the most part.) I don't think it will really "come back" as a popular music choice, but I am happy many new and old bands are still carrying the banner of true, fun thrash metal.
Many of those bands you mentioned did change their sound, but are more thrash than anything nowadays. You didn't mention Sodom, which really haven't changed that much (many criticize them for their punk-ish nineties, but they ALWAYS had a punk sound to them, because that's what Tom likes since he was a kid-it also makes them fun at times, as with Stalinorgel, Ausgebombt, etc.) Sodom, Kreator, Destruction, Tankard, etc. keep playing thrash to this day, and are quite unashamed of it. Same goes for many of the American bands you mentioned. Anthrax CAN play Thrash when they want, but they don't want to too much, for some reasons (perhaps they don't like thrash that much anymore themselves.) Earth on Hell is one of the most "brutal" thrash songs that Anthrax has ever done (you could put Schmier on that song's vocals, and it could "pass" for a modern day Destruction thrash song.) But they just don't go ahead and do a thrash album, which I know they have it on them to do, if they had the desire to do so (I know being popular plays a role, unfortunately, as they have to please more fans now, but as I mentioned, I just believe they don't want to go full-on thrash by their own choice.) But I also respect those bands that have risked being more or less faithful to their roots the whole time, instead of just riding a temporary rise in popularity, nostalgia, whatever you may want to call it.
It is important to mention the younger thrash bands, that inspired by the older bands' music, has put some incredible thrashers that put some of the original bands almost to shame. No kidding you, just hear a few of them with an open mind, and you'll see that many has surpassed or at least, equaled that fun and intensity of original thrash. Were it not for some of this newer, more exciting younger bands, perhaps this current trend of thrash "semi-popularity" wouldn't exist in the first place. These younger bands, along with some of the "faithful" of the older bands have kept the genre more or less "afloat"-but always remember that thrash is not a trend, but a music that lives in the heart of its fans-thrash never died for those who always cared for it. These younger bands care for it, and I applaud them for just playing whatever they want (in their case, Thrash metal), and not what is supposed to be more "popular".
BTW, I do like Anthrax new record, but not as a full-on Thrash album, which it isn't. It's also not fair to compare them to Kreator, etc. because they have never been that type of band. But yes, like Kreator and others, many bands were kind of lost, as in "what do we do now?" kind of phase during the nineties. It's great that Anthrax produced a good album with a few great songs in it, especially that amazing Earth on Hell-Joey really saves the album, IMHO!
All respect to all of you, I hope my opinions don't offend anybody. Feel free to disagree in peace.