It's more likely that STL closes completely and loses all air service than AA moving its ORD hub to STL. STL is a declining, sad city much like PIT was 30 years ago. O&D to LHR is practically non-existent compared to ORD. Similarly, PHX will never be a major TPAC gateway; the O&D is at much larger LAX.
Internet know-it-alls have repeatedly claimed that Parker will abandon ORD and surrender the market to UA, which is possible, but very unlikely. That would enable UA to build up to a ATL-sized operation at ORD. I suspect that the government would not let UA take over the AA real estate at ORD if AA abandoned it. Probably see a lot more LCCs and ULLCs at ORD instead.
Anyway, AA isn't going to build a hub at STL and reduce ORD. Anybody selling that fairy tale to co-workers should face a random drug test.