The University of Southern California and UCLA aren't going anywhere, and USC wins a lot. That takes a lot of the edge off of wanting a pro team. You also nailed it that in LA if you're winning, the fans are there, but if you're not winning, they're nowhere in sight.
This could be the first big test of whether the next meltdown will be in pro sports. The NBA has been moving quite a few teams. As you mentioned the NFL has always been good at keeping a slight shortage of teams. Since the last expansion, though, the threat of moving doesn't carry as much clout. The Bills are playing games in Canada. Jacksonville has never had a big following, and the NFL wants to be in LA a lot more than LA wants the NFL. With the roof of the metrodome caving in, the Vikings will need a new stadium one way or the other. If not LA, what city is there that wants a team that could support one? Possibly San Antonio, but I wouldn't bet on it. They have the Alamo Dome.
The thing is, though, the tax bucks ain't there right now, so the Vikings will either play in an existing facility or? Seriously, if they don't work something out in Minnesota or LA, where is there in the US that is big enough for a pro team that doesn't already have one?
Sacramento? Portland? Salt Lake City? Seriously, yes I get your point. The market is becoming saturated.