In Volvos online service literature (SB or Tech-net note, nowhere else) it's now listed that Volvo recommends replacing it at 70K mile intervals. That's really not bad for any engine considering what they do but I personally think that info should have been added in owners manuals as well and included with the 70K mile service.

In the "good ol' days" (years back), many believed that thermostats be replaced every two years so they've made progress.

Every engine made needs them replaced sooner or later or they'll likely become problematic, not just Volvo. The OEM T-stats are good, probably better and longer lasting than what the aftermarket T-stats will last without problems but only time will tell with those. For what it costs (about $15), I wouldn't gamble with an aftermarket part like that unless the dealer is too far away or has non in stock (my opinion).