Processes were a form of robust virtualisation once. It’s just pushing the problem down another layer. There are vulnerabilities in virtualisation layers too.
Was it ever robust? Or just that there weren't yet so many exploits?
Qubes OS v4+ does not use typical software virtualization methods. VT-d hardware virtualization it uses was broken only once, and it was done by the Qubes founder: https://en.wikipedia.org/wiki/Blue_Pill_(software)