I guess the truth is somewhere in the middle?
But math don't lie. If premades have a 70%+ win-rate as most report, then if you queue in during one of their sessions and you are not on their team, you have an average chance of winning below 15%. For some players that is highly demotivating. KK beats AA more often than that.
They of course can be beat - 70%+ is not 100% - and I agree it would be more enjoyable if people grit their teeth and attempted to do so, but we have to live in the real FL world.
As to how skilled meta premades are, I'd suggest they're very good at doing something that isn't particularly complicated. The members must also have a massive threshold against the tedium of simplistic repetition.