Kimetsu no Yaiba definitely isn't going to just quickly end and disappear, this isn't like Ass Class where everything's out and finished a year after the manga. The Demon Slayer anime still has at minimum 2 seasons and a movie left, which means at least 3 more years of content, and Ufotable's not just going to randomly stop making it considering how profitable it's been.
For perspective, Kimetsu no Yaiba I believe is already the 2nd highest selling manga per volume of all time in Japan. Bigger than One Piece, bigger than Dragonball, Astro Boy, Slam Dunk, Touch, everything except Black Jack, which people mentioned before apparently has multiple somewhat debated numbers? Even the movie's one of the highest grossing ever. Heck, even its opening theme is the biggest song in over a decade.
In 10 years time, we're going to start seeing a whole generation of new authors whose main inspiration was Kimetsu no Yaiba, who got into manga because of it. What the short term influence will be, who knows, but it will absolutely have a large influence later down the line.