and when you are an outlier, the comprehensive system doesn't work
Possibly, though I would dispute that this is necessarily true. I think that other characteristics of the individual school, individual teachers (plus the curriculum and exam system) are far more important than the pure fact of whether the school selects on academic ability or not.
But even it was true, it doesn't make sense to design a whole school system around the needs of outliers. You have to design something that works the best it can for the majority.
The vast majority of those 20% are not outliers, and there is no reason why the can't be well served in a good comprehensive school. And teaching the true outliers with those top 20% is in all likelihood not going to make all that much difference anyway - no different from being in a top set.
I was (arguably) an "outlier" once upon a time (top marks in my year of 300 at Oxford kind of thing...). I wasn't socially all that happy at my middle-of-the-road comprehensive. But I have no idea if I would really have been any happier at a selective school and to say that the system "failed" me in any real way would be ludicrous. There are certainly far, far bigger problems in the education system than a lack of optimum stretching of bright kids.
Yes I chose both my children to grammar schools. Would I have been happy if they had been abolished and my children couldn't go? Yes. But me opting out wasn't going to make any difference alone, so given the choices available I wasn't going to not send them on principle, though there were more highly principled and less pragmatic members of my family who thought I should.