Along with growing weekly breakevens are growing ticket prices. I think I paid $45 for a front mezz for Phantom in 1988.
Another factor in a show's running cost is the crew cost. For instance Avenue Q ran forever because of a rock bottom weekly breakeven. It had no extra musicians and just as importantly, nothing flew. They had no flymen expense.
Proof ran over two years partly because nothing in the set had to be moved by a stagehand.
The moment anything moves in a show's scenery, crew costs go up.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher