Why should the pullback in SJU matter to AA or WN as far as who flies where? SJU operations began when most of the island airports could only accomodate rjs. Now that a lot (if not all) of them can handle double-aisle size a/c, there is no need for an rj operation in the Caribbean.
If WN wants to fly to STX or any other of the Caribbean islands, their 737s can easily reach most, if not all, of them from FLL--just as AA is reaching those same islands with its 73s from MIA.
We (aa) probably won't be flying to the islands for long. Now that JB and WN are going to be flying there, I 'spect that AA will give them the market, like they have so many others. Shrinking to profitability. We are AMR. We will succeed at it even though countless others have failed.* First, we are going to outsource the maintenance, then the airport operations (gate agents and the like), and finally we will outsource the flying. We are going to make millions!
*And, if we do fail, it's because we pay the employees too much. That's the only thing that has ever been wrong with this company.