And I believe you on how hard the reliability/durability challenges must be in engineering these things — I've seen what the kids do to them.
BTW, I think the mechanisms themselves are no small part of the interest; kids don't just get to see whatever phenomenon is being demonstrated by the device, they get to poke at the thing that does it and try to figure out how it works, and that's a lot of fun for a curious kid; there are layers there.
According to what you've written here something close to 100% of those touchscreen exhibits should be broken. Are they?