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.
I believe it's actually easier to cope with what kids will do (banging it, trying every nook out etc), compared to many adults putting more force than needed on common mechanism or button or whatever as they figure it out.
But ultimately, it's about wear and tear.
Now that both adults and kids spend their days on screens, and are looking to limit their exposure, it suddenly makes less sense to have them in museums.
According to what you've written here something close to 100% of those touchscreen exhibits should be broken. Are they?