Here's a better comparison to pro athletes. Their work output is winning games. How do they get good at (and stay good at) that? Is it by playing real games for points?

That's a part of it, but only a small part. They don't get good at the thing mainly by doing the thing. They get good at it by training to do the thing.

An NFL football player does a ton of things other than playing in games. They have practice scrimmages. They do drills like throwing, catching, running patterns, tackling, reading quarterbacks, stripping balls, picking up fumbles, etc. They work with coaches on their technique. They watch film. They spend many hours in the gym and on the track building their strength, speed, cardio, and stamina.

Yes, it's true that your software skills will atrophy if you don't use them. But that doesn't mean your skills have to get worse and worse causing you to eventually quit the job. It means you need to set aside time to maintain your skills. It may no longer happen automatically as a side effect of your work, but it can happen intentionally instead.