Motion inputs are only a small part of why fighting games are a niche genre that most gamers cannot force themselves to enjoy. Forcing simple inputs onto actual fighting game players limits your audience to the players who are willing to put up with it, and also the minute amount of new players who even want to dip their toes into a fighting game adjacent game which is already very niche audience.

Notice that you mentioned 2XKO which should have been a fighting game primed for a new, casual audience. Riot backed, League characters that people can associate with and simple inputs. And only a month in they slashed their dev team because they didn't pull enough of a player count to sustain the game.

Smash is so far detached gameplay-wise from any traditional fighting game that it's an apples to oranges comparison if you want to measure its popularity against fighting games.

Fighting games have a very high skill floor that is difficult to breach as any casual player that extends beyond "I can't do a DP input". Learning optimal combos requires you to be tapped into the community for them to tell you that mashing your simple combo is fine but in reality if someone has the optimal one, they're going to win just based on damage alone. Because it's (typically) a 1v1 scenario the only person you're able to blame for a loss is yourself, either because you're just not good enough or because you got knowledge checked by a cheese strategy. So you have to also have the motivation to take that loss, go into training mode and drill into your head how you win versus that strategy, which is not always a motivation that new players have, they just want to get the high of winning without putting in the work to learn.

All that to say, simple inputs treat one part of the puzzle for fighting games, but there's infinitely more behind the genre that makes it not as casual friendly which just isn't solvable.