That's probably the reason the Koopalings are no longer used in more canon games now. They're more like classic characters that don't need to be soiled with modern developments of today's games. Can you imagine bringing back Wart or Smithy? It would either be the same old thing again, or else they would tweak the characters to make them more interesting and people wouldn't like the change from their original appearance. Still, I think the Koopalings could have been in Mario Parties in place of the koopa kids, you know, not important, but still present and accounted for.
Also, every other character has a sort of sidekick, and naturally Bowser does better with just one instead of seven. Another pair like Peach and Daisy, Mario and Luigi, Wario and Waluigi, DK and Diddy, etc.