With the title, the playwright, and three well-known actors (one of whom is widely beloved), it should come in with a very nice advance for a play. It's going to be tough to fill the Shubert, but if the reviews are good, it could do very well there. Even if it doesn't quite fill the Shubert, that can be fine. If it plays to 80-85 percent in the Shubert, the producers will do very well.
If it doesn't get good reviews, it probably will still run a bit, though that theatre might feel pretty empty at some performances.
Having said that, I do agree that they should have gone for a smaller house. The actors in
Private Lives a few years ago were known to have been very unhappy in the Rodgers, as well they should have been. I didn't like that production a whole lot and I don't think I would have liked it more in a smaller house, but those who did like it probably would have liked it even more in a smaller house.
On the other hand, Maggie Smith in the same play didn't seem to have problems in the same theatre in the 1970s. And I think the Smith production wasn't amplified, unlike the last one.
I do worry about this
Blithe Spirit though. I love the play, but I have very mixed feelings about Blakemore's past work and I think Ebersole is way too old for that role (and I'm not sure she would have been a good choice for it when she was the right age). I have hopes for Lansbury and Everett, though, and Atkinson's a classy addition to the cast. Ruth is the seemingly unshowy role that can steal the show and it wouldn't surprise me if Atkinson manages the trick.
As for
Spamalot, you can see at the links that it filled the Shubert very nicely for a long time:
https://www.broadwayworld.com/grossesshow.cfm?show=SPAMALOT&year=2005 https://www.broadwayworld.com/grossesshow.cfm?show=SPAMALOT&year=2006