So a bit of disconnect in what makes a grotto exhibit.
Does an open top grassy enclosure of hundreds of square feet qualify as a cave?
Is it about no fence or glass barrier for visitors or is it about being a cave?
Can Brookfield's large
Great Bear Wilderness be called a cave?
Dallas'
Giants of the Savanna has no fence or glass viewing but is it a grotto?
Are you really just talking about Hagenbeck's barless enclosures?
Are you really talking about fabricated rockwork to keep animals in a space? (As @
zooboy28 stated in the referenced thread)
So why might there be all those tall (very costly!) rock walls around the back and sides? Certainly there is nothing immersive about them. And you can't hope to "modernize" the concept if you don't fully apprehend its function.
(Yes, I am asking you to think this through -- which both of you are well equipped to do)