We don't know that.
It's certainly possible nothing goes in there for a long time (and it wouldn't surprise me, as I mentioned), but the Play! Pavilion sounded like a complete waste of space that was potentially going to be outdated by the time it opened. Building garbage doesn't improve the park. I have a feeling Disney realized this (that it was going to be outdated at opening) and that's one of the reasons it was cancelled.
Also, if they did eventually add attractions to the Play! Pavilion, they would have needed to fit the pavilion's "theme" to not feel wildly incongruous. That means they'd likely have been terrible attractions, since the pavilion itself was a terrible idea.
As for Innoventions -- the loss there is the building; not the specific exhibits that were previously inside it. The building could have been used for anything.
Again, adding capacity isn't inherently good no matter what. If it was, no one would care about putting Zootopia in Animal Kingdom, or if they decided to build a Wall-E ride on Main Street USA. I don't think it's a good idea or helpful for Disney to spend money on wasted space.