I'm not getting that vibe at all about Shazam. I don't think it has flop potential in the slightest. People seem buzzed to see it.
The trailer looked fun, so it's tonally consistent with Aquaman. I actually think it's pretty much the perfect movie to be the follow up.
I do agree that Captain Marvel is at best a "see it once" type movie. I think it has really nothing to do with Endgame except that Sam Jackson said that Captain Marvel can/will time travel, so that explains how she can appear in Endgame without aging.
I predict that Captain Marvel's appearance in Endgame will be a dramatic splash (kind of like when Thor arrived in Wakanda to face Thanos), giving her a notable heroic moment (like Wonder Woman in BvS), and be more talked about than her own movie.
It might have been a better strategy to release her solo movie AFTER Endgame to capitalize on that like Warners did with Wonder Woman.