I think you sort of answered your own question.

If you are unable to book any sooner then 2 to 3 weeks out and want an aisle seat, then that is hardly Delta's fault that the popularity of the flights doesn't leave the seat choice of your desire. I can imagine that it must suck to be stuck in the middle for such a long flight.

Maybe DL should leave more aisle seats to its highest elite flyers? maybe they already are and these routes are just medallion heavy flights? who knows?

I'd say keep checking the seat mapper up until the time you check in. Hopefully something opens up and if not, then it sounds like you might be finding yourself booking on united.

You might want to possibly also consider the fact that China is tops among areas that are currently experiencing a tremendous come back in air travel.

FWIW, I just checked the DTW to PVG flight for 2 weeks from today and sure as what the OP says, no aisle seats available. Hmm, makes me want to book a seat for this flight, I bet the chances of an op-up are excellent!