By having rewards there when they succeed, you know, like most games do. Failing is part of PvE, but the major flaw is that M+ rewards you generously even if you fall short. See: 1hr+ completion keys for Myth vault, hero track and gilded crests.
Auto-queue M+ would need to not reward failure or else all it does is perpetuate it, like people who queue for LFR and AFK because someone else will try for them.