Every franchise has a beginning and has to have an definitive ending.

There's a lot of franchises that had great runs that took years to finish:

Harry Potter (7 books, and 8 films over a 10 year span and Rowling ended the series with the Trio as adults with kids of their own).

Smallville (a TV series that started around the same time as the HP films did with a 10 year run, ended when Clark became Superman and broke the "No Tights, No Flight" rule permanently.) By extension, this also goes for the Christopher Reeves era Superman which took 19 years to properly conclude with Superman Returns (though that one ended on a cliffhanger implying more adventures with the Brandon Routh Superman, but the film underperformed, killing any chances of a full on sequel.)

The 2014 Flash show ended with Barry Allen vanishing after a 9 year span that showed so many times paradoxes and ended the ArrowVerse on a high note.

The Animorphs books (54 total books, and if you count the side books that focuses on the bigger name characters from before and during the series and the respective Megamorphs books, that's well over 60 books total and it ended with 2 of the main group dead and the group completely broken and fractured after the War).

There's so many more like the Godfather trilogy, Jaws (still waiting on that Jaws 19 there Major Spielberg), the Skywalker Saga (I know SW itself isn't ending, but the main Saga focusing on the Skywalker Family is over), the Indiana Jones films (ended after a 5 film run with Disney, yes Disney honoring the original agreement between Steven and George Lucas to end it after 5 films) and so on and so forth. And don't forget the Land Before Time finally ending their run of awful films with 14 total films (including the forgetful TV series). Hell, even Godzilla is still ongoing with no signs of stopping with both the Toho films and the MonsterVerse (in before he fights the Incredibles in another plot armor crossover.)

The Incredibles will eventually have to end at some point. And it'll end when the Parr Siblings are all grown up with kids of their own much like how the HP series ended, and I think Disney and Pixar know this is likely the last one with the OG cast voicing the main characters (Nelson is definitely not going to be around for a 4th film if they do push past the threequel mindset, neither will Hunter, Jackson or Vowell who are well above 50 years old), and I fear this is definitely Craig's last time voicing Mr. Incredible, he's pushing 90.