macOS unable to open any non-Apple application

This is the future Apple wants I guess, you don't really own your hardware, you simply have a limited license to use it under their very strict terms. It's just a matter of time before Macs become just like iOS.

It might get worse: now that they're switching to their own SoCs, they might even block APIs and allow access only to certified parties.

Basically Final Cut Pro and Logic Pro might forever be faster than any 3rd party software package by having access to IP blocks that aren't exposed to other developers complete with signature check to prevent reverse-engineered use...

If they really tried that, wouldn't the DoJ bring an anti-trust case against them? That's exactly what Microsoft was doing in the 90s, using undocumented internal APIs for their own software that let it run faster than competitors'.