My initial instinct was that the extreme temps of summer/winter would be the bigger lifespan shortener, but I suppose dust would start killing fans and clogging air intakes compounding the temperature problems.

This is pretty much it. Constant temps are more important than lower temps. Within reason of course.

Modern equipment is pretty tolerant to high intake temps, due to lots of very poor datacenter designs. It's pretty common to see servers in older co-location facilities at the top of racks seeing 95F intake temps if the on-site staff don't really care about rack blanks and the like. These can go unnoticed for years and simply chug away with their fans at high speed.

The thing is though, there are no temperature problems

All equipment is within the operating specs