The problem is multi-faceted I think:
1. thrash bands in the 80s made their best music back then (in pretty much all cases)
2. the legacy thrash bands are still around, making music that is almost always worse than what they did before
3. there was a time when there were no newer thrash bands around (I should know, my band was one of the very few doing it during those lean years)
4. now there are tons of thrash bands and albums released.
5. it's hard for the newer thrash bands to get noticed or any traction while the legacy bands are still around and getting all the attention
6. why should the new thrash bands deserve any attention if their albums aren't as good? few have shown they can compete with what was done in the 80s
I still think there are too many simplistic views towards thrash metal.
Things today can't be the same as what they were back in the 80's and 90's so therefore the metric of new and popular must also change.
I don't believe it's harder for newer bands because the number of ways they can become known has increased ten fold. The ability to research and find new bands is a whole lot easier. And the ability for bands from way out places like Upper Buttfuck East to find an audience can actually be targeted and driven.
Sure the old style of play well, get noticed, get an advance, make an album, tour, rinse and repeat worked well but if you were outside the big scenes you missed so much. Many German bands who were huge at home didn't make it big in other parts of the world until the world became smaller and easier to access. We had a huge, thriving scene in Melbourne in the 80's and many of the bands we played with could get gigs 3-4 nights a week, but the world scene for us didn't really open until Metallica toured in about 89, despite other US bands touring before them.
As for bands today, new or old, competing with what was done in the 80's well that's purely subjective. In the 80's it was new, it the 80's we hadn't heard a thousand bands doing the same thing. 40 years is a long time and if metal didn't evolve and bands of 2020 were still playing the same 80's stuff people would be getting sick of the same shit different band music. It would be like listening to the any black metal album on repeat for a month.
People have also changed and so many now lead with the negative. "I'm not going to like this", "their last album sucked so this one will", "he hasn't written anything good in 30 years", etc etc. It's hardly surprising we get so many negative reviews when the outlook so many have is shit. I'm not entirely innocent of it, but I do remember in the 80's we'd be excited to hear new bands, we'd play a set, then rush to the bar to watch the next bands set. We'd rush down the record shop to see what was released each week. We'd listen with open ears to whatever shit the guy at the record shop would let us preview, then we'd buy what we could and take it home and talk about it. I still try to do that with most albums be it new or old bands, it doesn't always work, for instance I love shitting on Anthrax, but I'm still going to listen to their new album in the hope that it's something good, not just assume it's shit, listen to it expecting it to be shit, then walking away saying it was shit.
New or old, I don't give a fuck, if it's thrash I'll give it a go. There has been some brilliant shit come out in the last 10 years from new and old bands. If others want to think thrash's best days are behind it I do feel a little sorry for them.