I suspect not one person who loves the grammar school system failed the 11+ and went to a secondary modern.
I've been in favour of the grammar system ever since it was scrapped in our area a couple of years before I would have taken it. I was a straight A student at primary school so would probably have passed it - a third of the kids from our primary went to the grammar. My parents fell for the hype about how "everyone would get the grammar education" as the grammar was converted into a comp. Result was 5 years of absolute hell in a crap comp, being bullied every day, property stolen/damaged, assaulted, etc., and finally escaping after 5 years without a single O level. The ex-grammar was over-run by the kids from the nearby closed down sec-modern - teachers were having nervous breakdowns, the school was vandalised almost daily. Instead of bringing everyone "up" to grammar levels, it dragged everyone down to the failing crap level.
Once I'd escaped, I vowed I'd never send any kids of mine to a hell-hole like that, so, yes, I was in favour of grammars even before I had children, despite not going to one myself.
My sister in law couldn't believe how intransigent I was. She had been to a good comp and had done pretty well - nothing spectacular, but a good set of C/B grades at GCSE which meant she could do the further education she wanted and get a half decent career. She had her children first and was aghast when I suggested they look at the local grammar and have her kids take the 11+. She was convinced the local comp was just as good - fell for the lie of "bright kids do well anywhere" and because she hadn't experience the comp from hell, she couldn't imagine how bad they can be. So she sent her kids to the local comp, which was Ofsted rated very good. Big mistake, which she bitterly regrets now. Both kids crashed and burned. Both started with excellent results from primary school, both left at 15 with a string of D&E grade GCSEs after bullying, crap teachers, etc and daughter who self-harmed. She is now the first to tell me how right I was to be sceptical and really wishes she'd gone down the 11+ route.
So, no, it's not just those who benefitted themselves from a grammar education that are advocates of it. It's also the ones who suffered from crap comps.