But people actually do do this, if for some reason a PC is overheating or the built-in fans are making too much noise. A big fan running slowly moves more air more quietly than a bunch of little fans. I even wired an external DC fan into the PC’s power supply once so that I wouldn’t have to plug them in separately.

My experience is that this also builds up a pretty spectacular amount of dust inside the case, so regular vacuum cleaning is called for.

Whole home hepa hvac filter on the intake side?