Exactly.
I mean, Junior himself was pretty boring to fight in NSMB for DS. He was a mini-boss in each world, and the same basic strategy applied each time. Jump on the head and GG. Galaxy was an entirely different story, though. He appeared 3 times on his own, and each "fight" was a different one (if you count when he just summoned fodder to fight Mario on his behalf "fights").
So, honestly, there's nothing that could stop them from doing likewise with the rest of the gang. And hey, if they actually gave them written dialogue (like Jr has in Galaxy), it would be another "first" for the original 7.
Besides, I'd like to think that, for all that they went through to not re-introduce the 7, but also give them brand new, revamped designs? That should imply that we should be seeing more of them, now. And shoot, short of like maybe a future iteration in the Mario Baseball series or whatever, how many other venues would be present to utilize the Koopalings?