Because the UNIX based OS are based on PDP-11 architecture with the compromises required to fit in such systems.

Whereas already in the 60s, we had hardware systems like the Burroughs b5000 that could be fully programmed in Extended Algol and Espol, with zero Assembly.

The idea, as I see it, to base modern OSes in better older architectures that were lost to the mists of time.

Actually the mobile OSes, with their memory safe languages seem to already be moving into this direction.

>these OSes design years ago are old and therefore bad

That's not the link's thesis. Equally flippant but more correct would be to say "these OSes were written in memory-unsafe languages because hardware limitations demanded it back then, therefore they are bad"