Of course it is necessary, but this is all on them for deciding to ship Showdown and King of Cards at the same time. They clearly did not have the resources to meet their stated deadlines. Therefore they should have focused on King of Cards, completed that - then done Showdown in a future patch. Particularly since such a game type requires lots of play testing and balance. No one is suggesting to ditch it completely. I just don't like waiting "several months" after having already waited for two years since Specter Knight so they can add a feature that is kind of a throwaway.
The problem is they have made a marketing decision, probably to leverage the Amiibo functionality and they recognize that Showdown is not enough on its own to juice sales. That's because you are buying Shovel Knight for a Megaman like side scroller, not for Smash Bros. 8 bit. It just feels like that time the Mighty No. 9 devs announced that one delay due to "online functionality", something nobody cared about. And it rings hollow that they also blame it on wanting to ship a "complete game" for the physical edition when Shovel Knight has already had physical editions that got patched for new content.
Complaining about extra time for the huge amount of free content they're putting out is a pretty bad look, dude. They could've easily just put out the playable characters promised in the campaign and been done years ago - but instead they've spent years they didn't need to making full fledged, highly polished new games and giving them away to people for free. That good will earns them a bit of slack.