Not that I feel this is particularly relevant to MHA, but they're doing exactly that with Naruto right now.I like comic books, but they have limitations that don't apply to manga. Mainly, they're owned by DC rather than any particular writer. I don't know how it works at Shonen Jump, but is there any doubt that if they could have continued Dragonball without Toriyama they would have? Or any popular series.
The point is that Horikoshi could keep it going for as long as he wants to.And that is HUGE. The reason Batman has a never ending fight with Joker isn't because it's narratively appropriate, it's DC needs to keep making new stories and they throw writers at the Joker wall until something sticks. If they end up with a bad joker story, no big deal, because there is always going to be a new one, and just using law of averages, something good will eventually stick. This also means that there is no way an author can actually end a villain. That's why the "why doesn't Batman just kill the joker" never really work, because Batman could personally see that Joker is launched on a rocket to the sun, and he could succeed, and we could personally see the Joker disintergrate on panel.....only for the next comic to go "Oh, that was just a body double". At this point, authors just leave their endings ambiguous so their stories finality isn't rendered moot by whoever they pass the buck to. So it's just a fundamentally different set up to most mediums.
I personally think you're misreading the curvature of the arc. To me the story is just getting started, and we have only seen the end of the origin story. Midoriya now has to man up and step out of All Might's shadow; that's a beginning and not an end. He's still very very far from being The Number One Hero. Hell he's not even the top of his first year class.All Mights loss of power felt consistent, as has much of the side character development. So I definitely feel we are working to an end point, it's just that I feel it's approaching sooner than expected. The only way I see this working otherwise is if we are introduced to new major players pretty soon.