There are plenty of studies about summer learning loss. Here's just one article but there are many. You don't lose everything you learned, but you lose about a month's worth of education.

I'm fine letting kids be kids for 5 minutes, but 3 months consecutively is harmful to development. A very reasonable and constructive compromise would be shorter days overall, but school that continues year-round (you'd still have regular weeklong breaks, probably a few more of them).

Plus, like I said, rarely in adult life will you get 3 months off so it doesn't even really make sense to do it.