From a non-local perspective, shows are crucial to BGW.
The only reason why anyone hours away from BGW would travel to BGW is because it is so different. Nobody I've known cares about flashy new rides and stuff (they usually don't have any idea what new rides are opening or anything), but instead always want to go back to Busch because of the differences between them and every other park. Shows are a good part of that, because things like Celtic Fyre and OktoberZest are so unique in the theme park world because of their dedication to the theme. Things like DarKastle, unique animal opportunities, Pompeii, the hamlets. All the things that make Busch unique are what the people at my university, 5 hours away, know and care about. I'm an hour from Great Adventure, yet still, we all are hyped to go to Busch Gardens.
All of my friends here have never even been to the place, born and raised in New York and such. Yet they have all heard of it from their parents or somewhere else, and know it is super cool and unique. Never once have I heard anyone besides locals to the park give even half a shit about new rides, ESPECIALLY not off the shelf ones.
I personally believe BGW's success lies in not just keeping the locals coming back, but keeping that uniqueness strong, so people feel the need to go there, and it continues to be talked about.