well, i mean mankind wasn’t alway around, man existed for less than a million year, so humans have not always been around and will not always be around. So yes will there be 30 realms then? Human bhavas will not be able to incarnate, just like they did when men where non existent. So the 31 realms division isn’t valid anymore and wasn’t valid 2 millions years ago.
But you don’t have the answer obviously, so that’s fine, nevermind