Every show will end at some point. FiM has with nine seasons already a very very long running time. Other successful cartoons don't get that many. Star vs Forces of Evil, Gravity Falls etc have less than nine seasons. That the number of seasons and episodes are limited is just part of my argument I made. ]\
The world building is highly appreciate from my end and there should have been more. Actually, with the ending of season nine I think this is also needed.
I didn't say that I they should I haven shown only the Main Six stories. You need to keep in mind that the student six are special characters. They need a lot of screen time to be really well established. While there have been already seven seasons for the main six, the Student didn't have that. You also cannot drop the Main Six completely out of the picture and even more important you need to show their interactions, how they are different, have different character progressions and so on.
There are so many background characters which needed already more time to be shown that introducing new character which needs to be establish is very challenging. There are many examples, but let's take just one: The pillars. They have been introduced in season seven. We see them once more in "A Rockhoof and a Hard Place" and "Friendship University" and beginning and ending of season nine. To say that they have been underused is an understatement. Even more important they share with the Main Six and Student Six they same role. I'm unsure how to put it down, let's say that the "Friendship is strong with them". Here much time would have been needed to establish and to develop these characters. Considering the ending of season nine it would also have been also very very good to show more about heir homelands.
And this is just one example, there are many more characters which needed more time: Spike, Starlight or the CMC. I'm still very sad that the CMC never get a bigger role with growing age. This is one the one hand understandable because lack of time. On the other hand: if you remember that hey became tutors in season eight then this would have been a good opportunity for the show to develop and connect them more.
I'm not saying I want only main six. I'm saying that this feels like a spinoff chance for a new show or movies and it would also have been justified from my point of view.