I'm not a SUPERFAN of this movie and haven't read the book, so maybe I'm the wrong person to respond to this, but no, I wouldn't like it as much. But that's because the movie's mainly about a widowed mother trying to help save her family, like that's the main draw of the movie itself. What's Justin going to do in the new colony or whatever? Literally who gives a fuck, I don't.
But that's only if Brisby is just there as a footnote. I don't get what the point of that would be. I mean, if there was just a movie focusing more heavily on the N.I.M.H. stuff or like Jonathan's life before getting married etc., that could also be interesting. That part of the movie was pretty compelling. I dunno about focusing on building a society specifically though.
I guess I just don't see what the point of having Brisby around only for her to do pretty much nothing would be.