The fans, disks and power supplies will most likely wear out before
the mobo components themselves. If you're not overclocking, IMO the
only thing that would normally prematurely kill a mobo would be dirty
power, a dusty environment (which can cause fans to fail or not work
properly, overheating the mobo), or high humidity.

We have a two of Pentium (I) servers at work that have been running
pretty much 24/7 (give or take a week or so) since '95/'96. I've had
to relace HD's, fans and power supplies (and one PCI video card), but
zero mobo components.

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Bob