People have this mentality that there should be individuals who carry every game, and the team doesn't matter. If you can't carry, you're useless. That's the mindset. In truth, individual players carrying the game should be occasional and accidental.
You're right, it's going to drive more and more supports away. As it is, it's hard to get a game if you queue as anything but support or fill because supports are in short supply.
People who think like that should probably go play fighting games or something. It's fine to feel like that in 1v1 games. But MOBAs require teamwork for a team to succeed, so until they put aside their individualist mindset, those people will continue ruining games for their teams.