They’re the faction that is least defeatable at the least.

This I totally agree with, at a certain point it would just be too much of a pain in the ass to deal with them, hence why I think they’ll end up just being a neoliberal hellhole confined to the California valley.

Is the interior particularly important in the event of a war? Big empty space. As long as what they need to maintain the war effort is placed in more defensive locations it’s actually not important territory.

Not really aside from fresh water (there is a separate water table east of the Rockies see image below), air bases, and nuclear silos. There’s arable land as well but I don’t think that’s as important considering the breakup. The most important factor probably being the Nuke silos, and aquifer which will get more important as time goes on.