From a box office perspective, Foster is replaceable. Jackman is not. He was able to sell out arenas around the world for a concert series when he's never released an album. He sold almost 35,000 seats at Madison Square Garden for 2 shows that grossed $4.2 million. No other Broadway star could come anywhere close to doing that. I would say no Hollywood actor could do that, either.
Jackman in The Music Man is like Bette Midler in Hello, Dolly! - it seems like a perfect fit for his persona.
There is no one else who could generate the kind of pre-sales for The Music Man that Jackman can. Rudin will wait for him as long as he needs to.