DL miles don't expire, so OP should certainly continue to collect them. The AA/US program is changing, so if OP doesn't already have a significant number of either AA or US miles, I'd credit AA flights to AS for the flexibility. For *A, Aegean would have been a good option to try, but that's changing now too, with no obvious good alternative although TK is one of the choices being discussed.

DL generally has the network and performance that I would value, but it makes sense for the OP to consider alternatives for every trip and think whether just picking the cheapest flights with reasonably convenient schedules makes sense rather than trying to earn miles.