Demon Slayer's story is short but sweet compared to its shonen predecessors like Naruto and Bleach, meaning it has several story arcs or sequences perfect for a movie rather than a full anime season of 12–24 episodes. This makes Demon Slayer's anime rather experimental, but too much experimenting might be a bad thing for Demon Slayer in the long run.
While the Mugen Train tie-in movie was an astonishing success in Japan and elsewhere, that doesn't necessarily mean that Demon Slayer should rely on movies so much to succeed and keep anime fans engaged. Movies like Mugen Train have their place, and other anime series can borrow that idea too, but overuse of Demon Slayer movies actually says something highly negative about the core anime.
Demon Slayer Shouldn't Use Movies As A Crutch
It's already clear that blending a broadcasted, episodic shonen anime like Demon Slayer with tie-in movies can produce great results. The Mugen Train story arc was largely self-contained and was too short for a conventional anime season, thus the Mugen Train movie, but that may be an exception going forward. Already, it's been suggested that the Swordsmith Village arc's epic conclusion should have been a movie too, and the producers already made the To the Swordsmith Village movie to bridge Seasons 2 and 3. Between Demon Slayer's actual and hypothetical movies, the franchise is going too deep with tie-in theatrical releases.
Turning so many story sequences into movies, and suggesting that even more should have been movies, doesn't flatter Demon Slayer at all. In fact, making so many tie-in movies suggests that the core anime isn't exciting enough or good enough to keep fans engaged, so Demon Slayer uses movies as a crutch to wow fans in the theater with loud sounds and flashy visuals. This is best done with careful moderation and restraint, and looking back, Mugen Train was the only tie-in movie Demon Slayer has needed so far.
Other tie-in movies, such as starting Season 3 with a movie and hypothetically ending Season 3 with a movie, actually looks like a desperate move. The core Demon Slayer anime should not rely on more than one integrated movie to excite fans and generate buzz. Mugen Train should be a unique and decade-defining success, not a blueprint to keep hammering anime fans with theatrical releases to keep the anime afloat.
Demon Slayer's Anime Should Stand On Its Own
To a degree, the Demon Slayer anime is setting a great example for integrated anime movies that bridge the series' seasons, with Mugen Train as the best example. The Chainsaw Man anime could do that with its Reze arc between Seasons 1 and 2, and Jujutsu Kaisen might do something similar in the future, too. Otherwise, though, the original concept for anime movies is clearly the best one after all: to make tie-in movies a fun bonus adventure for enfranchised fans. These non-essential movies are spinoffs that truly stand on their own, and the original anime shouldn't rely on movies more than once – at most – to succeed.
The core anime series should speak for itself, such as with Jujutsu Kaisen, My Hero Academia, and That Time I Got Reincarnated as a Slime. Those three franchises all had one or more tie-in movies in recent years, but none of those movies helped prop up the original broadcasted series. Those anime series confidently stood on their own, and their movies were merely bonus adventures for fans, like a victory lap. Granted, those anime series didn't have story arcs short enough to be like Mugen Train, which helps explain why they don't use integrated movies, but even so, those anime series handle movies better – and Demon Slayer should start following their example right away.