I have been using a Pi 4 as a desktop computer for a few years (didn't have anything else) with an microSD card and without any fan, heatsink or case. Haven't had anything problems. Obviously, this depends on your environment, but it worked fine for me.

Completely depends on what you're doing. If you're doing a lot of sustained compute, or doing graphics, then yeah you're gonna want some cooling. But it's a useful little machine for all kinds of tasks which don't cause sustained high power consumption.

I've had an rPi4 running a copy of a forum and server (for reference) in one of the fancy aluminum cases which passively cools for a couple of years now, no issues.