As a taxpayer and citizen, there are good reasons I want every kid to achieve great education results. The fact that it is probably impossible, is irrelevant. I want that. This is why we have universal education in most countries.
Despite the fact that the results aren't what they could be in an ideal world where every student is motivated, the results are much better than any place where education is not universal.
Yes. And that's why universal education has been success and why a modern idea of education – "let's find out everyone individual needs are, adjust to these, show how cool it is to think critically etc" – is such a disaster.