Thrash evolved in many ways beyond simply paving the way for extreme metal. Megadeth, Dark Angel, Prong, Coroner, Pantera, Voivod, Anacrusis, Fear of God, Meshuggah, Stone, Nevermore, Depressive Age, Mekong Delta, etc all had plenty to say and do differently from one another. The thrash bands died largely due to lack of label support more than there simply being no further room to push things.
I don't know why you keep shitting on punk music either, which is without a doubt one of the most diverse and creative forms of rock music. Black metal itself was basically predated by hardcore like Charged GBH and post-hardcore/noise rock like Blind Idiot God and Ham. As far as I'm aware, there are still bands pushing the envelopes of punk's many children and grandchildren. Black metal wishes it had anywhere near the breadth of punk music.
Malokarpatan is retro-heavy/speed metal based on their new album, probably 15% black metal at most. Not familiar with that other one, but outside of dissonancecore stuff like DsO (which arguably has more in common with Immolation/Ulcerate/Gorguts than Burzum or Emperor), black metal hasn't shown any musical progression over the last couple decades afaik. All the essential tropes were established in Norway in the early 90s, mostly by just Mayhem themselves. You're right that the sound isn't as pigeonholed, but only because it's black metal fans that do the pigeonholing. Meshuggah writes songs with one riff? Totally not even metal, let alone thrash! Wold writes songs with one riff (or less)? Ambient noise black metal brah.
