Every show will eventually jump the shark, I do not expect the show to last forever. A bit of cartoon 101, back during the 80's it was common to have a very large cast, much of this was part of the marketing tied into the show as each character would have their own toy and through the series every character would be featured, even if it was even just one episode, while the rest were rotated periodically with a few having more memorable moments than others. While this was great for advertising, this did not make for great story telling and television.
Show creators have learned over time that a small cast for a show to focus on allows for character development and for the audience to build a better connection to the characters. This does leave room for side characters to be introduced and even share the spot light with the main cast from time to time, but the main cast will always have presence. FiM has been doing this, much like other modern shows airing these days.
Having some episodes that bring some light to background character is not a bad idea, if done right, but such episodes will have members of the mane 6, or the CMC, present as they need to be, even if it just one or two them, in every episode.