An issue in this discussion is "what is a focus city?"
One
description is
In the airline industry, a focus city is the name for a type of airport. These airports are not a hubs but are important for one airline. An airline has daily flights from the Focus city to many destinations that are also not its hubs.
some have been using the concept of a city / airport with more service from more hubs.
If one uses the first description, the focus city is operating like a hublet -- allowing more shorter, spokes in the network -- a fit for UX type flights.
Looking at a 600-mile air-distance (90 mins gate to gate tolerable UX distance IMO) from UA hubs, you get the following map
There are few coverage gaps
-- Northwest -- SEA has service from multiple hubs, eastern Washington State, upper Idaho are sparsely populated
-- Southeast -- this is mostly a Floride issue and multiple larger FL cities have multiple hub connections.
One advantage of a deep SE hub would be better Carribean coverage