As for Miis, they're still important to Nintendo and their only avatar character(that isn't specific to a game series like Pokemon Trainer or Villager are). They clearly are not going away, and at best, are less of a focus than the Wii and Wii U. They are not just recent, but hardly are irrelevant. There's no way they're going to be cut from Smash any time soon, especially not the Switch version. If they're dropped by the next console, that version might drop them from Smash, but it's not worth dropping anyway, since they give people what they really want, the ability to essentially play as any possible character. Which is basically a major catering to the fans of the series, as it's well known that some character cannot get in(unless they originated in a video game, which inherently makes them eligible no matter what. Not likely, but still eligible). This is Sakurai's perfect workaround when he can't add every single character, and also allows people to play as non-video game characters just fine. Sure, they aren't allowed in all modes(which depending on the design of the mode, is entirely justified. With Anyone has things like copyright issues and issues with people making inappropriate Miis, and that's enough for Sakurai to be worried. He's completely justified in his worry and doesn't want to take changes. As for the Smash Party(why can't I remember this mode's name?), the Miis are your player character, but it's like Pokemon Trainer in the Pokemon games, you don't entirely play as the game, although in this case, you have less control).
K. Rool is actually pretty irrelevant at this point, but also is a retro at best nowadays. His last game appearance was on the GameCube, which is more than 2 systems ago, which is the cut-off time for Virtual Console games. Depending how you look at that, if they're eligible for VC(2 systems before the current one), they're retro. I'd say he ultimately counts as that, overall. He doesn't have any proper modern appearances either. Plus, "retro" has two meanings, a design style(see: Shovel Knight), and the system it's on(anything 2 system prior to the current one or generation. Which makes stuff like Dreamcast retro). Another example of the difference is Pac-Man and Sonic. Neither are from retro games, as they're active franchises with works on systems even after the GameCube/GBA. However, they're both designed with their retro abilities/appearances as the main focus. Sonic barely has any modern moves(his Down Air is extremely similar to one in Sonic Battle, but that was also done when the GBA wasn't a retro system either, but even then, it was one of his only modern moves to date. Even the Homing Attack isn't modern, originating in Sonic 3D Blast, a Genesis game. Although to be fair, this isn't exactly common knowledge, as it was used as a staple of the modern 3D games for Sonic/Shadow and similar playstyles).