That's coasting though, or at best flow. Neither state makes you better. The entire point of making something easy is to be able to build on it.

I don't think this is accurate. In basically every single game or sport with measurable outcomes, doing things are relatively simple to you (and often enjoyable) endlessly - drives improvement.

If you want to make the argument that it's muscle memory in e.g. shooting freethrows in basketball, then you can see the exact same thing in doing chess tactical puzzles. There's even one successful learning method called the Woodpecker Method where you endlessly repeat over the same series of tactics working to get the time it takes you to do them down to essentially instantaneous. And it works excellently for improvement, and I obviously don't just mean improvement at doing that set of tactics.