I would love to be wrong, because I think Great Comet is a fantastic show that I would happily see multiple times if I lived near New York, but I just have my doubts about its staying power. It seems like the hope was that bringing in a mainstream star like Josh Groban would draw crowds and that, along with stellar reviews and possibly some major Tony wins, would allow it to keep running after Groban and the original cast leaves.
It's worked, up to a point, but Great Comet hasn't turned into the must-see show of the season. Instead of Hamilton, or anything close, it seems more like a combination of Hedwig and Fun Home, acclaimed musicals that have a cult following and a relatively short Broadway run. Hedwig steadily lost steam after Neil Patrick Harris left, and it must be a much cheaper show to stage than Great Comet. And Fun Home also has a small cast and is easy to tour.
For many people, myself included, the show is the star. But it doesn't have an easy hook to draw audiences, and the mix of mainstream Josh Groban fans and an avant garde downtown musical was always a weird fit. (The irony, for me, is that I knew almost nothing about Groban and was more interested in seeing the musical, and I thought he was fantastic.) Oak will draw some curious Hamilton fans, but even if that bit of "stunt casting" works, they have to do it again in three months.