When Ryanair and Southwest serve six continents we can talk. Even domestically, count the airports served by AA and AA Eagle vs. fewer than 100 destinations systemwide by Southwest.
Oh yeah, I'm not suggesting that American should revert to just one type. Just that they definitely might want to simplify their line-up, for the same reasons that LCC's choose not to operate different planes.
For a large airline, a streamlined fleet could look something like this:
Regional: Type X
Medium-haul: Type Y
Long-haul/International: Type Z
One oddity about American right now, is that they've got the A320, B737 and MD80 in serious numbers, all three aircraft having a very similar profile. Admittedly, the MD80 is the odd duck and on the way out. But still, that means right now that they gotta keep up with FAA notices and organise maintenance and parts for all 3 types, while an airline that only has either the A320 or B737, only needs to keep parts for one type in stock, and only keep maintenace engineers trained for one type available.