IMNSHO, the view that provided they get the grades to go to university all is well is wrong - catastrophically, tragically wrong, and I see the effects of it every day.
Here's what often happens to clever children who don't get stretched: they turn into perfectionists. They develop a self-image in which part of the definition of being them is that they never find academic work hard. For them the only respectable challenge available is to get 100%. And they shouldn't have to work hard to get it - other people have to work hard. For so long as they can meet that, and don't have to deal with actually finding things hard, all is well, the school regards them as a success story, parents think vaguely "s/he worries too much" and may notice the perfectionism, but don't really think it's a problem.
Then the child reaches university, and (with luck) is faced for the first time with a genuine intellectual challenge, with concepts s/he can't immediately understand, tasks s/he isn't immediately confident of being able to do to a high standard. Some young people take this in their stride, relish the challenge, learn the skills, do fine. Many fall apart. Not only do they not have the practical skills they need to deal with something that's hard for them; far more troublingly, they feel there's something terribly wrong with them and with the universe if they can't do it. They decide they're doing the wrong subject, or they're in the wrong place. Often, they stop doing any work, because that gives them a reasonable explanation for why they aren't getting high marks, and is far less threatening than having to face the risk that they might work hard and still not get high marks, because who would they be then?
People get ill, and miserable, and fail because of this effect. Lots of people.
Over the several decades I've been teaching in universities, the entry grades of the students I've worked with have risen quite dramatically. It seems to me that I see this phenomenon far more often than I used to, and I think this is why: I'm now seeing the students who've never been challenged before, instead of the ones who were pretty good but still did experience needing to struggle sometimes in their later years of school.
It makes me angry and it makes me cry, and it's why my child isn't at a state school even though as an academic it's not easy to afford fees. For goodness sake, do whatever it takes to get your child appropriate challenges, in their area of strength not just their areas of weakness (or they'll learn that they have to work in subject X but never in subject Y, and won't be much better off when they go to university to do Y), while they still have the level of individual attention that school can provide and they still have you nearby to support them emotionally. Please.