No, they aren't called grammar schools.
If your analysis is correct, their' success' is based on never having to do anything difficult. We could all succeed on that basis.
All children need to be educated, including those with 'issues'. If such pupils are all booted out of the so-called 'best' schools, they go somewhere else instead. That somewhere else is then derided as a failing school because it has a high proportion of difficult children and consequently, on average, lower outcomes.
It's a bizarre apartheid in education. Of course, your average parent only cares about their own kids. The government's job is to care for all kids, including the ones who are a pain in the arse to teach. But let's not pretend that 'good' schools are 'good'. They're like a craftsman who carefully selects the very best pieces of wood to work with and burns the rest.