You're assuming that the soundstage lot isn't used. When infact it's been an active production site for multiple television and films in the past few years alone.Because they keep on building hotel rooms and they know people want to enjoy attractions and they want to put buts in seats?
In that logic they wouldn't have build Kong and Velocicoaster, both empty lots that no one complained about...
I agree that there's a lot of sense to potentially having this ampitheater be turned more into a proper flex space. It'd give Universal a chance to use Soundstage 33 for park development (possibly Pets, as it can fit quite well into that space).