What actually would be the benefit of supporting more cores for players?
Currently on consumer hardware the AMD Ryzen 9 7950X3D as being the choise on high end systems has 16 actual cores. Intels i9 14900k has 24 cores (16 "efficient cores", 8 "performance cores"). Threads don't count as the Threadripper even has 192.
Many people complain they cannot run the game properly on their 2017s hardware with GTX GPUs on 8 cores, so people using a Threadripper or other server CPUs for gaming is the niche of the niche and most games, consumer simulations or applications don't even come close to utilize even 64 cores, so saying the game uses "just" 64 cores is pretty dull imho (even in the video they appreciate that it's actually using 64 cores). I don't even know a single game which comes close to this level.
"they should just turn it into an api compatible fully bespoke unity replacement at this point"
"just" probably means another 2 years of development plus testing cross platforms ( keep in mind that the same build still has to run on Xbox Series which means additional testing of something bespoke on hardware not as easy to test using the dev kit only (ask Asobo how that went when it comes down to WASM support which went just fine on PC but not at all on Xbox OS...)) on a core team of maybe to who deal with that stuff to accommodate the needs of 0.005% of all players for the next 10+ years.
Outside of university and research things have to have a real value and have to be economic. It would be just a waste of development time and resources to care about this just to say "we can run on an infinite amount of cores for basically no reason!". More important is to make the simulation run as smoothly as possible for the other 99,995% of players because they actually pay the bills.