This is a fascinating question indeed, but I'll take a shot at it.
Show Boat- I've been paying attention to the recent thread, and while I think that it will definitely see some sort of concert or very limited run for its hundredth anniversary, I don't anticipate a full-scale revival in all its glory. I just don't think that producers would be willing to take such a gamble with a century-old show that doesn't have a massive fan base under the age of sixty.
The Music Man- While I absolutely adore this show and I hope it lasts forever on Broadway, I just don't feel this one. If the Jackman/Foster revival manages to survive the pandemic, I anticipate that that production might be the last one on Broadway. The fans are dying, but I don't think the show will cease to be. It's perfect for community theaters, and I imagine it will exist all over the world in that capacity.
Finian's Rainbow- This one doesn't need too much explaining. The only revival it has had since the sixties lasted only about fifteen weeks. I wish that community theaters would give more attention to this show, because most young people don't even know it exists. I think this one might die out in the next twenty years unless it gets some major TLC.
Singin' in the Rain- No one is begging for this one to return. I don't even see local high schools in my area giving this show any attention, nor has the local community theater ever done it. However, I do believe that this show can be saved with massive stunt casting. Get Darren Criss and Harry Shum in the leads and you've got something.
Then you've got all the ones that community theaters and schools did back before the twenty-first century, shows like Babes in Arms, Me and My Girl, Li'l Abner, Camelot, Over Here, etc. They just wouldn't have the audience, and I don't think they ever will again. It's quite a shame, since Over Here holds a special place in my heart, and I've never seen it live.
My wild card for this list is Sunset Boulevard. Norma is my ultimate dream role, but I'm not quite sure anybody is screaming for this show to return, especially since it was just on Broadway. We'll have to see if a movie (starring Glenn?) ever gets made, because that could make all the difference. I, however, am confident that this is another show that can be saved with stunt casting. Get Lady Gaga or someone else of massive stature in this role and we know it would sell. The question would then become quality.
I apologize for writing a novel here, but I have a lot of thoughts. Does anybody disagree with these?
"See a picture of a woman wearing four years of confusion like a scar."