Everyone saying the show will be ruined if it keeps going on, all you need is fresh blood to not halt the creative process, and to prevent a mass take-over of the original canon Hasbro needs to dictate stuff now and then, which is going to happen because they love doing the Buy Our Toys meme and stuff. If something doesn't work they will be hecka concerned, its different from a network hiring cartoonists for shoddy wages for two decades, over-working them and burning them out.
Also pretty sure season 8 was confirmed, and 'til 2019 at least' it might as well be a given it'll get another season. Movie hype is a thing, they will need more seasons to 'Buy Our Toys' longer. And if the writing is bad they can swap writers because its not just an overworked Faust or something like other networks would've done.
I would sooner worry about SU or Rick n Morty becoming bad relying too heavily on one or two minds. Also, Star has [intentional] plot, I doubt that show will continue excessively. Rick n Morty freestyles, so it could become bad, but Rick n Morty might be like wine, and get better with time as the writer's suffer more lol(prolly take a road like family guy if I had to guess, it seems like its going to be picking up for season 3 which had a good start). Idk. SU tho, for sure, if it gets dragged on too long it feels watered down and they run out of ideas to drag it on, then by the time they have to wrap it up its going to be a mess and no one will like the ending.
And why I feel that way for SU and not Star Vs is because Star really matured season 2, I mean yeah it has filler, but its not plot filler, SU has plot filler. And SU feels like some of its moments are forced(blue/yellow diamond's song for instance). Like that meditation one, granted its cult psychology, maybe it is realistic for someone as easily emotionally disturbed as Steven. Oh, now I see why people call it SJU. Convincing extraneous family to break from norms, not going to school, getting emotional during training when it should be a hobby or something, empowerment for doing whats right. The Gems are unpopular radicals that recruited people. They 'truly care' about people. But granted that perspective has its flaws too.
So, to not be mildly off-topic. Um, MLP is going to keep going, its good they kept a multi-writer format so I don't think it'll stagnate, I think the flavors(approach in content I mean, not quite 'style') may shift over time for it. Also the show is self-conscious, look at S7 E2. I doubt they will fall into some terrible conundrum. Also fantasy has ALOT you can work with. But its going to not change too much, due to Buy Our Toys and demographics. So the show might cycle out old viewers and in with the new, which is prolly already happening considering alot left the fandom after season 2.
In which case the movie might become the mid-point of Gen4. It has to be well writ tho for that to happen, idk if they can pull a Faust, Faust would be good for a time like this I think. Also, that's only assuming the movie doesn't bomb.
Unless of course the art style of the movie is to brace us for a new gen's new style. But considering how much time it takes for that style, its doubtful that its the case.