This has been discussed somewhat ad nauseum and there's an extensive thread on it.

In general the Florida market is somewhat price sensitive and while Spirit vaporizing may open the door a bit further, there's still a relative infestation of LCC/ULCCs in Florida that's probably not going anywhere. The question is not would UA be more competitive in the SE USA but does UA need to be more competitive in the SE and would they make more money competing aggressively there vs. where they could invest the same money in other markets (where UA can command higher fares and RASM).

AA is wounded and probably couldn't mount too significant of a defense of MIA for too long on its own, but if UA tried actively chasing DL market share on their home turf I imagine it would turn into a relatively bloody (red ink) mess for both with UA probably being the first to back down. Plus MCO isn't really that well situated for US domestic connecting traffic (being way out of the way of for anyone who's not either starting at or ending in the Florida area) and I wonder how much utility it would bring to UA international destinations.

What I could see is UA doing more point-to-point flying to MCO selectively (but at a far lower level than a hub) and potentially even some point-to-local-point Express flying to backfill for 3M... But 3M has been gone for a while and UA doesn't seme to have really noticed so perhaps that's not a capacity hole they see a reason to fill.