My US account from the 90s (I wasn't a big flyer at all back then, on any airline) went dormant long ago. When I tried to find it to credit a flight earlier this year, the (very friendly) US phone agent couldn't even find me in the system and said my long-dormant account likely wasn't even transitioned to the new IT system (which we know to be SHARES) following the US/HP merger. No great loss for me though, I can't imagine that old account's MM counter would have totaled more than 10K BIS miles.