I agree. However, it's also true that believing in magic will set higher expectations (or... different expectations). You'll be expecting more from life (in the case of Santa, you'll probably be expecting him to show up with a few presents; in the case of Hogwarts, you'd probably expect to get that letter). If those expectations are met, then it'll be all just as though you lived in a world without magic as a non-believer. If they are not met, then you'll be just as disappointed as you would be if you were disappointed in a world without magic. So, what I'm trying to convey here is that, ultimately, the magical world would become the normal world, and after your childhood years you wouldn't be any happier than you are today. In fact, you probably quickly forgot that Santa didn't exist once you saw you had presents under your tree (or I hope you had some
) anyway.
But I don't disagree with you! Ignorance may very well be bliss in a lot of cases. But then again, some people prefer the bliss of knowledge (totally made that up), or feel that the truth is part of the pathway to happiness (or some **** like that!).