This is of course not including the yearly Unity subscription, where Unity Pro costs $2,040 per seat (although they may have Enterprise pricing)
Absolutely ridiculous. Many Unity devs are saying they're switching engines on social media.
Assembly usage is pretty minor in these engines. Tends to be for just a few very tight loops. It has to be redone for every platform, too. Assembly for x86-64 doesn't work on ARM. Hell, some things on 32-bit x86 won't even work on x86-64. You would never want to do more than a function of inline ASM here or there. It'd be a nightmare if you did.
That said, it's barely even touching on the complexity of modern engines. Unity and Unreal aren't just engines, they're a whole development ecosystem.