The really intriguing alternative that remains: Tampa. The Tampa Bay metro area has a population of 3.2 million, the 18th-largest metro in the country - larger than Denver, Charlotte, or Salt Lake City which all support substantial hubs.
Neither DEN, CLT, or SLC have a major international airport 85 miles to their northeast, or a regional airport 60 miles due south.
DEN has COS right about 60 miles due south. CLT has GSP and GSO within about 75 miles. SLC is more isolated but there's some service at PVU.
Still, the larger point remains in that the traffic leaks toward DEN, CLT, and SLC. Some of the RSW traffic definitely leaks toward TPA, but some TPA traffic doubtlessly leaks toward MCO and the huge number of flights and low fare seats offered there.
The really intriguing alternative that remains: Tampa. The Tampa Bay metro area has a population of 3.2 million, the 18th-largest metro in the country - larger than Denver, Charlotte, or Salt Lake City which all support substantial hubs. It's also one of the fastest-growing (nearby Lakeland as well), and has manageable low-fare competition. The housing is far more affordable than the Miami area. It's also about an hour from Walt Disney World, the largest tourist attraction in the country (soon to be connected by the Brightline train). And it's not just tourists - there are plenty of business travelers as well (United's core market). Tampa has been steadily growing its corporate presence - especially financial - and it's the center of Florida's High Tech Corridor.
As has been pointed out repeatedly already, Tampa lacks the geography for domestic connecting traffic that the other three cities you mention all have. This means we're really looking at a hub connecting domestic markets to Latin America and the Caribbean. There are two problems with the proposal.
1) It's largely redundant for UA so they'd be cannibalizing some of their own traffic. IAH works just fine as a connecting point for about 2/3 of the country geographically. IAD is a perfectly good connecting point for the upper midwest and northeast to these markets, so really all that's new traffic for UA is southeast to international markets.
2) TPA demand to those markets is far smaller than from MIA or ATL. The O&D side of the proposal is weak and the flights will be even more reliant on connecting traffic than AA or DL. A high cost airline running a small hub for low yield traffic will get crushed.
Also, I'm not sure why you highlight housing costs versus Miami. Are you making a point about disposable income? Miami appears to have higher per capita GDP so without researching it further I'm not sure there's any advantage here for Tampa.
Florida's High Tech Corridor isn't Silicon Valley. It's a grand sounding name attached to an economic development initiative not dissimilar to what most other states have set up.
It is a mistake to think you can solve any major problems with just potatoes.