Like when graphics stack and drivers were "imported" into the WinNT kernel as an optimization for the user-space version. And thus began 30 years of "another week, another graphics subsystem crash/vuln".

Indeed. These are gigantic mistakes. The quest for performance is diametrically opposed to having a secure system. There will always be a price, sometimes a big one for having a system that enforces proper boundaries. In my opinion: if something can be a process then it should be a process.