The Marvel movies have the advantage of being pieces in a larger puzzle, and that is one of the appeals.
How many people, who had probably never otherwise ever heard of Thor, went to see the movie because of the end-credit scene in Iron Man 2? I'm guessing a lot. Hell, the only reason Coulson was even in IM2 was to lead the audience to Thor.
Do you think there would be this much anticipation for Captain Marvel if it weren't for the teaser in Infinity War? And look at that trailer-- Narrated by Nick Fury? Check. Familiar villains? Check. Coulson? Check. And of course, they give us a brief shot of the pager. Double check. They go out of their way to say, "Look! Here is the next piece of the puzzle."
However, the "Star Wars Story" movies just seem random and directionless, and I think that that is what led to the failure of "Solo" (Which I saw in the theater twice). That, and the extremely poor marketing, not anything to do with The Last Jedi.
I think you make a good point here. The Star Wars side films might be more important if they had clues and ties to the main sequel films. The side films could've been where we see Rey's parents, Finn's parents, learn Snoke's origin, etc. Marvel has discovered a great formula for how to tell side stories but still connect them to the overall larger story, even if it's just an end credit scene. But when you take the big surprise cameo in Solo, I mean, what was the point since that character's fate was already known by many of the people who would be the most stoked to see that character again? If that had been Snoke for example, it would've shed a little more light on who that character is or what he was up to, and made Solo more important to the new films, therefore more of a must see for sequel trilogy fans. The new films should be driving things, but because they aren't really that good IMO, and there's still a lot of love for the older films and characters, and deservedly so, Lucasfilm wants to serve all that need, however I think they've gone about it in a way that doesn't tie their films together enough, with the side stories being pretty stand alone, relatively speaking (I didn't see Solo but I've heard about it, and while the potential for a sequel is there for that film, it's still a film that has little bearing on the sequel films, from what I've read/heard about it). Even though I liked Rogue One the best of the Disney Star Wars films, I mean would it have been better to shift the story to trying to steal the plans for Starkiller Base? That way we could see Poe's parents perhaps, see a younger Poe, perhaps Max Sydow's character, more General Leia, learn more about the First Order and Resistance, etc. Rogue One, despite my feelings for the film, didn't need to be made. It scratched a nostalgia itch for certain for me, but it didn't really push the larger story forward. It was like filling in a blank that didn't need to be filled in. And from what I heard about Solo, that film also did the same thing, even more than Rogue One.