Interesting points. Capacity would be an issue for sure. Of course any additional capacity would be welcomed at WDW, even for those of us with 0 need, interest, or desire for such, but it'd keep those kids off the streets and rides, I'm all in
Those types of rides also aren't unique. You can get them tons of places, so I'd think it wouldn't draw people to WDW if they advertised "little kiddie" land, folks could get that cheaper and closer to home. They could do unique rides/attractions like the Alice hedge maze which would be cheap and serve all kinds, and you don't see those at the local county fair/small local park. I'm sure WDW could come up with all kinds of unique family friendly offerings, I just don't think it's a sexy marketing campaign that will draw additional park guests.
This said by someone with no children (thank heavens) - but would that actually make anyone plan an additional trip? If it won't move the needle, why would WDW do it?