To your first point, while MIA and PHL are strong international hubs for AA, I think you are right that UA's hubsities (SFO, EWR, IAD, e.t.c) are superior in terms of international reach. But then again, international has, as you pointed out, been a focus for UA for quite some time so I naturally expect that to be the case.
Domestically, is a lot more debatable, and it could go either way or in between depending on where you are based. For example, a lot of my travel is to the east coast, so having nonstop service to WAS, NYC, PHL, CLT, MIA, and a large presence at BOS is huge. But, I could understand if I was based on the west coast, and the only major options were LAX and PHX on AA(although the AS partnership offsets that a bit). UA's presence (or lack thereof) in the southeast is their Achilles heal domestically.
Back to the main point about focus cities though, UA has a lot of work to do in order to build up their hubs domestically, which in turn affects their ability to create focus cities. For one, UA has a huge problem in how big their RJ fleet is, specifically the CR2s. It obviously isn't an easy fix for UA, but it is something that impacts their ability to compete in non-hub markets. You can't build focus cities in cities where you don't have any point-of-sale strength, and the reliance on CR2s and other 50 seaters is certainly a deterrent for many business travelers.
In addition, there is a lack of frequency from non-hubs to some of UA's major hubs, including DEN, IAD, and LAX. As I mentioned previously, UA's route network from LAX is lacking, but even IAD has some major holes including MEM, SLC, MKE, OMA, e.t.c. Now obviously those destinations are not make or break for UA, but it shows that significant work can be done before attention is drawn to focus cities. Additionally, DEN is ideally placed to replicate some of AA's success with DFW, much more so than IAH. UA recognizes this problem and is expanding their frequencies/banks to IAD and DEN, but until those are beefed up more, UA has ample room to grow their hub service.
Finally, there is no where for UA to put a focus city. While AA and DL are usually the
#1 or
#2 of the US3 in large to mid-sized non-hub markets, UA is generally
#2 or
#3 . Plus the most obvious locations are taken:
AUS (DL/WN focus city already)
BNA (DL/WN focus city already)
SJC (AS/WN focus city already)
SAN (AS/WN focus city)
STL (WN focus city)
MCI (WN focus city)
RDU (DL focus city)
CLE, sorry CLE folks, it isn't happening
PDX (AS hub)