I think the suggestion in the OP has merit. AA, IMO has the best program for the 100k flier but probably the worst for the other levels. ON DL/US/CO you get unlimited upgrades at the 50/25k tiers. On United you get E+.

There are pro's to the AA approach but personally, if I was at the 50k level and had a choice of another airline where I flew I'd probably take it for the unlimited upgrades (AA is only easy option out of STX though).

Sean