The problem with Kirby's moveset is that he doesn't use enough from his source material in his general attacks. For instance, look at Shulk. His UAir is actually from Xenoblade Chronicles (apparently) and is called the Monado Purge, or look at Link's Downward Thrust (from Zelda II.) Kirby has a colossal range of different abilities that he could use. Look at, say, the Fighter Ability for his jabs, maybe the Beam Ability for his F-Smash, and other such ideas. See, his specials are alright. Inhale makes sense, Final Cutter could work (but then, couldn't something like a small Warp Star better reference better known source material?) Obviously he also has Stone (which is fine) and the Hammer (which is okay...) But, consider the source material. There's easily around a hundred different abilities that Kirby uses throughout the Kirby games, and yet how many of these said abilities are referenced in Smash? It's an odd choice, because one would think that Sakurai would take great care in making Kirby's moveset especially interesting and engaging, and referencing the source material with more moves. Things like the Fire Ability for a dash attack work well, but just having unoriginal kicks and punches for moves is kinda dumb.
Then again, look at Captain Falcon. His moveset is literally the exact same as it was in 64, and yet it's still effective. If they got rid of, say, Raptor Boost and Falcon Dive, there'd be uproar, because his moveset works.
Same with Game and Watch. Someone's mentioned this already, but he plays completely differently to how he used to.
We're not against character's movesets changing. If anything, it allows originality and relevance. Look at Link; the new design is good, but the key is that he still plays similarly; that is, he's still a projectile-centric swordfighter, which makes sense. Kirby still needs to be the light puffball that he is, but needs to maintain usability. After all, if Ganondorf's moveset were to be drastically changed in, say, 20 minutes, in a new update, to make it a better reference to the source material, what would Ganondorf mains think? Completely changing a character is something that the team at Nintendo have never really done, not fully, because it's a dangerous game to play. As such, we kinda stick to the moveset we had to begin with, with minor alterations along the way, mostly cosmetically, with some moveset changes over time in order to maintain relevance. As such, Kirby is not likely to change much from what he is; and, to an extent, nor are most characters in the Smash games.
That's just our opinion though, so it's not fact. Take what you will, and have a nice day