For me, Shovel Knight getting in is the first instance of a bandwagon character successfully getting into Smash. Literally the only thing he has in his favor is that he's the flavor of the month, and I personally think that that's a very dangerous road for Nintendo to go down with additions.
Shovel Knight has pretty much no relevancy to Nintendo, at least not that he doesn't already share with literally every other console on the market. He's currently this game's Geno, a completely irrelevant and unimportant third party who inexplicably got popular because the game they were from was good and that they were popular in their original game. Even then Geno has an advantage over SK, because at least he was a Nintendo exclusive character, whereas Shovel Knight, up until the very recent announcement of his amiibo, had MORE content in his game on every major console BUT Nintendo's. And just like Geno's support post-Brawl dwindled and died, Shovel Knight's undeniably will as well, and he'll fall into the pile of characters like Gooey, Midna, Geno, and Krystal, of characters that had large fanbases once but that fell off the face of the earth by the time of the next Smash.
Nintendo giving credence to obvious bandwagon characters such as Shovel Knight is a very, very dangerous thing when it comes to future Smash content. A Smash where all a character will need is to be in the right place at the right time and have their creators beg hard enough in order for them to get in, regardless of their importance to Nintendo or it's history. Doing this with a third party is even MORE dangerous.
And I say this as a massive Shovel Knight fan who stayed up until 4 am to perfect Plague of Shadows. This is a terrible choice if Nintendo goes through with it.
tl;dr: Adding Shovel Knight is about as bad as adding Geno to me - a character who has no relevance to Nintendo whose fleeting bandwagon support will ultimately die out and whose support now will be be just as laughable in the future as Geno's is to us now.