The OP isn't saying online multiplayer is a bad thing. Hell, online multiplayer is great.
The problem is games that say "You can play multiplayer, but even though your system can support up to four controllers at once, you can only play our game over the internet. If you want to play with your friends in the same room, you must each have your own seperate system/game/subscription." That is a major problem. Some of today's older gamer's best memories are that of yelling taunts at their friends while they're sitting next to each other after blasting them with a red shell in Mario Kart 64, or saying "how the fuck did you fall for that?" after planting a trip mine in Goldeneye.
Split-screen single-console multiplayer is dying out, and it's a very, VERY sad thing. That's the issue here.