Modern arenas have more than a 30 year lifespan. These aren't the bare, practically disposable concrete shells containing section after identical section of identical seats found in the old buildings from the 70s and early 80s. They're much more robustly built and have fundamentally better skeletons for upgrading. Sure, the suites may get renovated periodically. And the scoreboards may get replaced with bigger, higher resolution ones (that, ironically, actually weigh less than he ones they're replacing). But renovation, not replacement, is the way these buildings are going.
Most of them do have lifespans longer than 30 years however the arenas that are 30ish years old that are still fully operational and not in the process of being replaced have had $100MM+ renovations, with something being significantly more than others). The useful life an arena isnt how long it can remain standing, its how long it can remain profitable and desireable.
Its much easier, and cheaper, to keep a concert venue up an running that a venue that houses NHL/NBA team(s)