I don't think spin-offs of Kirby are meant to "evolve" Kirby, they are meant to innovate the platforming genre I think. One of the biggest part of the appeal of Kirby to me is this trend to innovate the platforming genre (I still think that Kirby Tilt'n'Tumble is one of the most ingenious games ever). 2D platforming is my favorite genre of videogames, and today Kirby is the only IP that tries to do something new with that, if you exclude some obscure indy games.
I appreciate attempts like Boxboy too, I haven't played it yet but I will sooner or later.
My only problem with Boxboy is that, it has a cool innovative gameplay concept, but it also removes lots of those aspects that make Kirby so awesome, like music, visuals, depth, world development etc. or at least so it looks on my first impression, and this may change since I haven't played it yet. But yeah, I'm fine with new IPs as long as they make them as good as the classic ones, with attention to little details (a thing that Boxboy, sadly, seems to lack of, taking inspiration from indy games like vvvvvv that are very simple and focus on just the core of the gameplay).
But one of the most iconic traits of the Kirby series is having spin-offs that use creative gameplay ideas, so I don't see the issue if they use the Kirby IP doing that; again, this is one of the reasons why I love Kirby, I'd be disappointed if they transform it into another New Super Mario Bros like series.
To conclude this, I want to state that I said I usually prefer the spin-offs because have original gameplay ideas, but this doesn't mean that I don't like the traditional games, I just prefer innovation over nostalgia.