The problem with multiplayer are the other players

. Seriously, there are times where you want to have control over your environment, to do stuff without having to worry about some idiot getting in the way, or being a good teammate. The key is good AI, frankly, GTA would be worse in multiplayer, just a bunch of idiots killing everyone in sight.
Games that depend on MP will be hard to keep on playing. Only the very best will still have enough people in a community to keep the game going. Most people will move on to the next big thing and games that are MP only will die out no matter if the SP still loves the game and wants to play. Can't find enough people, too bad, might as well say your copy is broken and they don't sell that program anymore. Gee, I guess that's like Final Fantasy 7 for PC, can't find it anywhere and that was a great game, SP only.
What the future of gaming will involve is great AI, not MP. If the AI can respond in a way that is at least challenging to a human without having to resort to too many "cheats" like having a thousand units to your one, then you will have a good game as long as the concept is decent. If the concept is good, then with good AI, you have a great game.
MP just substitutes AI for real people which pretty much equals the "optimal" AI. An opponent that can adjust and change tactics on the fly depending on what you do. Being able to play with friends is a bonus, but no big deal if it goes away, the real draw of MP is facing a worthy opponent that is roughly your equal. So if programmers could somehow improve AI to the point where it behaves like another human, then there would be very little need for MP. I suppose some people will want to test their skills against others, but if there's not that much difference from playing people or the computer, then most will simply just play SP against the computer. That eliminates poor sports who quit midway, annoying talkers, anal folks who must be the leader and want to be obeyed at all times, etc.