I totally understand your points, and I agree that the plot should be utilized better than it is currently. I think there are hypothetically better places for a large-scale theatre within USF and I honestly would want to see that prioritized above most other projects. While we have FFL however, I would much rather see them fill the current vacancy during standard operations and keep the theatre around until a replacement is ready. I believe it is simply too important for HHN, and in turn, USF.
HHN is an upcharge event and immaterial to park guests entering in April or June, but I also believe it is still a massive attraction for the park and it should be treated as such. It brings an absolute ton of foot traffic and money that I believe USF would not recieve otherwise, and is the primary way a portion of guests experience the park. I believe the live shows at HHN, specifically B&T and Nightmare Fuel, are incredibly important for the event's capacity, variety, and overall guest experience, and the event would suffer greatly if it was removed.
It stinks that they installed a load-bearing component into the unthemed, giant ampitheatre between two amazing themed lands, but until they fill that niche with another proper theatre space, I think it would be a net negative for the park to rip it out. To me, it's equivalent to replacing the Old and New Parade Buildings with Pokemon Land with no replacement storage space built in it's place. It's too big of a loss for a major seasonal event, which themselves are headliner USF attractions.