I apparently had a Delta account back in the 1980s or so that I'd totally forgotten about.
I had another Delta account.
For about 10 years starting in 2002, I flew NW exclusively (living in Mpls does that) until they merged with DL. After the merger, my lifetime miles were the sum of the DL and NW miles.
A couple of years later, my other DL account came to light. I asked DL to merge it into my existing one. Any redeemable miles had long since disappeared, but the lifetime miles were still there and were most of the ones I still needed to make MM.
Just a data point on how an apparently more legitimate airline handles a similar issue.