I'll start this discussion off with the OBVIOUS odd man out. It maybe hard for Spidey fans to agree on this simply because of their blind devotion for the web slinger, or simply because they have no clue on what's fact or fiction. The truth however, is that our friendly neighborhood wallcrawler is the least likely Marvel hero between the three current franchises to have a film series that lasts another decade or two. At least not before average movie fans get tired of seeing the same old characters over & over in every film they're watching. Since Columbia Pictures has a VERY lackluster roster of superhero stars under their umbrella right now, character variety is definitely not on their side. Simply put Spidey has no allies for him to team up with when dealing with the supervillains of his universe, & without an Avengers or Justice League team for him to claim all to himself it will eventually come back to bite the film studio in the rear end big time. More people than not won't tolerate seeing just solo heroes without eventually wanting to have them in some sort of collaboration ensemble film at the end of the trilogy, or whatever amount of films the studios puts out. Spider-Man in the long run will surely suffer for this since there's obviousely no one on his level who could team up with him, & get people to care about such a group in his film world. You can throw out as many potential character names within his universe as much as you want, but the facts are that there'll never be an Avengers or JLA type group in the movie world known as Spider-Man, & because of that his A level status on the big screen will diminish much sooner rather than later.
The franchise that has a MUCH bigger advantage than Spidey's when it comes to having a chance for a very long existence on the big screen would be Xavier's mutants. Although I've never been a fan of the X series I still feel with the amount of top level heroes, & villains at their disposal 20th Century FOX can still make that franchise relevant for many years to come. Plus, now that they're about to put their very own cinematic universe franchise into full throttle starting very soon it also gives the mutants more longevity. Lets be honest though about this "potential" super team up world between the two FOX franchises. It's clearly obvious that one of those film series can definitely survive a long time without the other, but the idea of FOX still bringing them together will still be a dream turned reality for most fans of both properties. The mere notion of the X-Men not only being a massive franchise of their own, but also having other X teams within their world clearly puts them on equal footing with Marvel Studios band of superbeings. When you factor in the world of the Fab Four, & how much of an impact they could have by intertwining their characters as well should make the competition between FOX & Marvel that much more exciting. However, if FOX doesn't get their act together when it comes to having X-Men's continuity make sense, or putting out a Fantastic Four movie that doesn't suck it won't matter how many characters they have at their disposal. Fix those problems first then people can have discussions about the X-Men franchise lasting a long time, or even "challenging" the MCU for box office supremacy. Wolverine might be the biggest thing they have right now, but even he isn't capable of keeping that series afloat without the proper direction that franchise needs to be headed in.
This last opinion might appear as if I'm rooting for Marvels' films to succeed, & the other two properties to fail which couldn't be the furthest thing from the truth. It's just that I absolutely love what Kevin Feige & his people are doing right now over at Marvel. So much so that I'm one of those comic book MOVIE fans (not COMIC BOOK fans) who's glad that Spidey & the mutants belong to other film companies. All I read from comic book fans on various websites is how Disney should just buy back everything so that Marvel can have all their characters underneath one roof. Two reasons why that plan would fail miserably. For starters Disney doesn't have to buy anything for anybody. The mouse house has always clearly been in the business of making money, & with these recent Marvel releases they have been doing just that...LOTS of it. So why would they want to spend a ridiculous amount of paper (which of course SONY & FOX would ask for) on Marvel properties they don't necessarily need right now, or ever for that matter. They currently are the owners of the 3rd highest revenue making franchise in cinema history with this MCU series. Both the Spider-Man, & X-Men series can't even see 3rd place much less be compared to the frachise that is actually in that spot. Spider-Man is all the way in the 10th spot which puts it seven places below Marvel's franchise, & three spots below DC Entertainments golden boy series...BATMAN.
What's worse is not only does the X-Men come in at number 15, but it's two spots below another Marvel property that currently only claims three films under its umbrella...Iron Man. On Sony & Fox's end the deal would be a mistake also considering how much they both would miss out on any future profits they could make from both those properties. That's why I've been convinced a long time ago that neither studio wanted to sell their franchises to begin with, & they never will. It doesn't matter though, because as long as Marvel has a huge line up of characters at their disposal which they always will, & great stories to put them in which they always will I don't see any momentum they have right now being lost anytime...this century.