@CRay
Edited, just to make clear that I was mistaken. Might have gotten confused by another article or something, will be more careful in the future. Also, by the way you phrased your comment it looks like there's around 100 chapters left until the end? What the heck, that's a lot. Which makes me kinda happy tbh if it's true.
@SuperOniichan
Well, my credibility is shit right now. But let me explain myself nonetheless. No, by the time a third season comes out there will probably be material to make 2 more seasons. In part because there was material that wasn't adapted in season 2 that might make it into season 3 even if they were chapters before 101 (Where season 2 ended). If the manga was ending soon, the third season would be enough of an advertisement for people to go and buy the last volumes... Making a fourth season unlikely to happen as there would be no reason to promote a work that has already ended... Unless the IP holder would want for that piece of work to stay highly relevant and get more sales which cases exist but are not that common.
Anime is just some kind of ad for the original works or the IP. It doesn't really hold value for itself and it's expensive to produce. Because of that, a lot of shows are incomplete and missing a last-season. Even popularity doesn't grant that a new season would be produced (NGNL can be an example). I think that's exactly what could happen with Kimetsu no Yaiba now that it ended. It might never get another season because the movie would be enough of an advertisement push for the people to go and read the manga.
In fact, movies are even better promotionals than anime in the short run, because the ticket sales actually report money to the investors while simultaneously promoting the IP. That's the reason a lot of those shows that didn't get a second or third season got a sequel movie instead, Kimetsu no Yaiba included (I'm thinking Youjo Senki, but that show is not that popular... Maybe Rascal does not
blablabla and The Dissapereance of Susumiya Haruhi are better examples...? and let's not forget my shoujo-ai thingy with Hibike Euphonium! and its three (two) movies in less than 3 years)
Something getting a complete adaptation like the NisiOisiN works (Monogatari Series, Katanagatari
which I'm crazy in love with) or a full on reboot to re-adapt everything (FMAB) are really RARE cases in the anime industry. I couldn't be happier if Kaguya-sama was adapted completely once it ends, but I gotta be a little realistic with those things.
Fuck... Sorry for the extension by the way.