It's interesting that the original production barely did star-casting after the OBC, despite the connections of Mike Nichols, Bill Haber, and the Python brand...Clay Aiken was a stunt as Sir Robin, and Robin Williams reportedly came very close to doing a short stint as Lancelot. It, like THE PRODUCERS, struggled after the OBC left, and replacements can be tough with ensemble-based comedy.
As others have said, there's no way this revival is successful (or even raises its money) without some major stars. And not simply a couple of C-list TV/stage actors...the box office difference between a Harvey Guillen type and a Josh Grisetti type is negligible.
Sometime I'd love to see a British ensemble company -- Mischief or someone similar -- revive the show in a scaled-down way and really lean into its homespun roots & ensemble vibe.