Besides some practicality issues that have been pointed out, if we have a 10-house line-up with 1 double-up, we have a 9-house line-up because you're giving up space in an SS. Tent 3 is no longer an option, and VillainCon would still cause congestion at the front of the park while also providing a smaller venue for a house.
The other argument is, "What gets deemed the headliner?" I can make a case for Fallout, Jason, *redacted*, and Five Nights.
Insidious seemed to have been the busiest house last year, and no one would have deemed it the headliner over GB or Quiet Place before the event started. If we had doubled the GB, it would have been a waste of resources.
The more practical idea is to diversify the line-up and spread the offerings out across the park.