If the spinoff were "let the CMCs grow up and take the place of the Mane 6", wouldn't it basically just be FiM:TNG?
i.e. There's no reason to do that unless they think there's more story potential with a fresh slate of characters and maybe a new premise (i.e. not Friendship). But unless they planned to rework the whole style of the show (or send them all to some new exotic locale or become a superhero team fighting crime or something), I don't see how it would be fundamentally any different from the show we have now. And of course if they did
change it that drastically, it would take an awful lot of getting used to—I couldn't say sight unseen whether I'd like it or not, but there's something very "perfect storm" about the gestalt of FiM as it is that makes me love it so much, and I don't know how successfully they could capture my heart by changing any of the ingredients.
(Also, I have a reflexive reaction to seeing spinoffs of that type as being the result of some falling-out with the star actors from the original series, so they whip up a new concept around the cheaper secondary characters. Unless we're talking about a Seth McFarlane style spinoff where you use the exact same art style and just swap in a new cast using the same talent and proceed according to the same formula as ever, in which case what's the point other than it worked for Seth?)
But jerrylee, you're absolutely right—I don't want them to even hint
at the Mane 6 getting older, because I have been absolutely destroyed by a number of fanfics that explore that concept and I need
the show to have them always young and immortal in their own way.