Today we mainly have

Unix

Linux

Windows

TempleOS

At the time; 1999 some of the choices were

Unix

Mac

Linux

BeOS

DOS

Windows

WindowsNT

I think that realistically we are just at a point where computers have matured enough that the operating system archituetures are largely irrelevant and creating new things is likely not to add value. A hammer is a hammer.

I would really like to see more things happening with ARM and RISC-V. Hardware had been incredibly stagnant until Apple went ARM. They seem to be the only company seriously doing this at the consumer level right now though.