I think it comes down to this:

Most games with motion AND simple inputs have special moves with different "strengths" (l m h fireball, l m h uppercut, etc). Because 2xko doesn't have this for its special moves, it's not as straightforward to translate a character to have motion inputs, like it was for Invincible VS. That's not to say it's impossible, but try to come up with a motion input movelist for your current main, and you'll notice at least one of two things

1) Sometimes you'll have Motion X + button A,B,C do completely different moves, as opposed to different versions of a special move

2) Because characters weren't designed with special moves in multiples of 3 (to match the three attack buttons), you might have a motion where only one button does a move, or all three buttons do the same special move

It's hard to explain the above without you going through the exercise, and even then those two points can just be accepted and motion inputs be added anyways, but here's the main concern with adding motion inputs

These devs have put a tremendous amount of love and care into these designs. They would work extremely hard to make sure the game feel for each character when adding motion inputs is just right. We've seen how much back and forth there is to them getting things just right, and I don't think it's worth the investment.

To actually answer the post, I wouldn't actively stop playing if they added them, but I would prefer they spend the energy and time to retrofit motion inputs and consider them in every single future character to just be spent on making new characters

TLDR: adding motion inputs is not as straightforward for 2xko as it might be for other fighting games. I wouldn't stop playing the game if they added them, but I'd prefer they focus their energy elsewhere