Memory pressure? You mean niceness?

Niceness just gives a program a higher CPU (and thus, RAM) priority.

Your system is still going to swap. The idling ram doesn't magically go away. That's how it works. If it didn't, you start experiencing bugs, crashes, and data loss, because there is no more room in the pool.

Just because a computer is "idle" doesn't mean it isn't doing anything, it is still performing work to stay on, composite your window manager, display it, run services in the background. Those services are still programs that are often vital to the operating system actually functioning, you can't just make that utilization magically disappear because you need more RAM because they tend to have lower niceness values than programs in userspace, even Windows understands that.

Yes, if you are swapping at idle then you indeed did do something horribly wrong, it is a bit comical that you don't realize that proves my point. The explorer.exe devs indeed did something horribly wrong, they released shitty half baked code because managers think more code = harder/better work when it is actually the reverse.