Despite being a thoroughly mediocre movie (but better than The Batman), The Lost City is also the most important movie of the year. Its success is so important for so many reasons.
Cinemascore is not always accurate. Bridesmaids got a B and legged it out like gangbusters. House of Gucci, at 3 hours and featuring a group of evil and unlikable characters, got a B+
It's true that even the biggest stars today are not known by people over a certain age. Honestly the average person probably won't know who Florence Pugh is. It's like they don't exist outside the film twitter/people under 25 bubble.
Watching The Lost City, the audience doesnt for a second question Sandra Bullock (57) and Channing Tatum (41) as a couple. She was also 10+ years Ryan Reynolds' senior in The Proposal.
You're right that they're both past their prime (and Clooney was never truly a draw) but the two of them in a destination wedding comedy is more appealing than the two of them in a financial thriller.
It doesn't have the adventure element that Lost City had. So they can't market it the same. And Julia is no longer as big a draw as Sandra. Julia had a bigger career peak than Sandra (it's insane how massive Pretty Woman and her string of hits were) but she hasn't had the same lasting appeal. I think it's because her brand of comedy is more sly, less broad and self-deprecating.