A typical desktop OS runs ~100 processes before you get to run anything. Even if you would delegate one core to system stuff, most programs are multi-threaded, so you are going to run out of cores quickly.
Is it a good thing to have so many processes that the user doesn't understand and often isn't even aware of?
If it's a security concern now, think of how bad of a problem it will be after AI is writing most programs that people use.