I don't know what kind of expansion PHX would be due for. It's primarily leisure and Western connections. If it were AA that merged with HP 20 years ago, they probably would have downsized PHX.
Phoenix is the 5th largest US city and PHX is the 11th busiest airport in the country, ahead of (for example) SFO, IAH, BOS, and MSP. It's actually an underserved market, particularly for international (though AF, CI, and JX have all recently begun service). AA has really failed to develop it, both in terms of infrastructure (arguably worst ACs in the entire system) and network.
In any event, the post you reference was suggesting that UA make a play for PHX traffic, which is really already happening. It's in the top 5 of UA's non-hub operations and offers easy one-stop international options via SFO, LAX, IAH, DEN, etc. We regularly fly UA to Hawaii, as AA schedule, pricing, and hard-product leave much to be desired.