Is that any different to separating them because some aren't "religious" enough for church schools?
Yes it is different, but also bad. I'd ban those too.
Anyway, we're not in the 50's/60's anymore. The alternative isn't a crap sec-mod these days. The alternative is a comp, and given how so many people claim comps are just as good, how can it mean "failure". Things have moved on in the last 50 years!
These are the words of somebody who knows nothing about the education system. The alternative is not a comp (even if it decides to call itself one). Comprehensives are schools which take the full ability range. You can't have those in a grammar school system.
And the 'comprehensiveness' of a school merey describes its method of intake. It doesn't tell you whether the school itself is good or bad. So 'many people claim comps are just as good' is an utterly meaningless statement. Each school is different.
Your paragraph about privilege and tutoring is utter nonsense too. No, a very low ability child will not pass the 11+ however much tutoring they get. But for the huge swathe of kids who are borderline ability-wise, tutoring and parental help will be what makes them pass or fail.
Add to that the fact that kids have good days and bad days, good months and bad months. They progress in fits and starts, regress a bit, catch up a bit, are affected by which teacher they have that year etc.