I think as far as crank replacement goes…unless you break something, or remove square taper arms a lot, a crankset should last for awhile. I guess with square tapers, constantly taking them off and putting them back on eventually causes the fit to loosen too much.
I tend to wear through bottom brackets pretty quickly though. Cheap ISIS drive models have teeny tiny bearings. I’ve ditched the ISIS drive cranks for a set of old 105s, so I get better life out of the cheap Shimano UNCs, but still, I consider sealed BBs to be throwaway items, given that I ride year ’round.
I’ve heard the new-fangled outboard bearings last longer, because the bearings are bigger (as their size is not constrained by the BB shell). But I’ve never actually used one.