A paddleboarding instructor recently remarked that "it's very easy to do paddleboarding badly." That statement applies even moreso to inline skating, ice skating, longboarding, cross-country skiing... all disciplines which at first glance appear "simple", but can become quite technical if you want to move beyond the surface.
In the context of keeping skiing-related SKILL in the off-season, which is what this thread is focused on, lessons are also a good bargain. No passes or lift tickets, etc. and a real prospect of not just maintaining but developing and improving skills that will mean skiing, when someone gets back on snow, can after a day or so of getting used to the remaining differences, be better than ever because of the skating work. (Or time on a MTB, etc. etc.). There are bike-handling coaches and instructors out there, too, even though lots of people may think they know how to ride a bike.
The reality is that very few people do activities in the off-season that simulate much at all the demands of a sport like skiing, so in the context of this thread, for most people their ski skills will inevitably atrophy during time off. If they want to keep or even in some ways improve skills and also their sport-specific fitness, they do have ways available, however. That's not going to happen at the climbing gym, because sports have specific demands and for skiing you need an activity with similar demands to skiing.