The best game moments are unscripted and happen in MP. No one cares what you've done in SP except maybe when you show off a high score or a video, or do something extraordinary like a speedrun but that happens to have an online MP component too, since the fun is comparing and competing with your friends and others.
SP will always be there, but will probably be even more niche as things move online. MP is good, it brings gaming more into the mainstream instead of the solitary, reclusive gaming nerd.