Star Wars: Episode IX is a movie that already feels potentially doomed to failure, at least in one respect.
That's not necessarily a statement on the quality of the finished feature - a film that, after all, hasn't even started shooting. (Indeed, it's a project that is still in the rewriting process, according to comments made by Lucasfilm president Kathleen Kennedy at the recent Star Wars Celebration event). In terms of objective quality, there's every possibility that it could be the best movie of the current core trilogy of films launched with 2015's The Force Awakens.
It's also not a comment on the financial success of the movie. After all, unless something entirely unexpected and dramatic happens between now and summer 2019, Star Wars will remain one of the most successful film franchises around, ensuring that the feature will at the very least recoup its cost, if not manage that many times over before it leaves theaters. The sheer notion of a Star Wars movie failing at the box office seems even more ridiculous than a Marvel failure at this point, much to the undoubted glee of Disney's accountants.
It doesn't even have anything to do with the now-confirmed May 2019 release, although already, fans are complaining that they've come to prefer the December releases for Star Wars movies - a schedule that's only existed since 2015's The Force Awakens, but which helped the franchise stand apart from the other summer blockbusters filled with sci-fi and superheroes.
No, the potential problem with Episode IX is its strange placement in the bigger Star Wars picture. It is, technically, a final chapter to the current trilogy, if not the entire "Skywalker Saga" that has run through seven of the eight Star Wars movies to date. Fandom had long considered the model of a trilogy of trilogies to be the length of the series, even before The Force Awakens was announced, so there's a double weight of expectations resting on it from the very start; it has to not only tie up the themes and plots of the past two movies, but also offer some level of payoff to eight earlier features, as well.
On the one hand, this isn't an impossible task. After all, both 1983's Return of the Jedi and 2005's Revenge of the Sith managed variations on this task for their own trilogies, with Sith attempting to tie together the two trilogies into an airtight - some would say, airless - whole. Both of those movies had an advantage than Episode IX lacks, however: they actually were intended as final chapters at the time they were made.
Episode IX, by comparison, will arrive before an audience who will likely know the release date, plot and cast of another couple of subsequent Star Wars movies; no matter how much it tries to offer a conclusion to events, audiences will know that it's not the end, or even the end for awhile. Instead, there'll be another chapter coming along, most likely in the next year. How can a movie successfully provide closure when everyone involved knows that the story isn't actually closing?
Given the success of the franchise, it'll be interesting to see if Disney opts for the one obvious solution to avoid forcing Episode IX to become the Schrodinger's Sequel path of offering a quasi-conclusion while leaving space for the inevitable sequel: announce that there's no trilogy structure anymore, but instead an ongoing narrative with a new installment every couple of years. If nothing else, it would easily answer the "What happens after the end of the new trilogy?" questions.