When it comes to plateaus, they seem to be mostly temporary blockages in learning that are very frustrating to people to have up until that point made a lot of progress. The reality is, any learning that occurs over a reasonable time-frame will have these stalls.
Why does learning stall? A lot of reasons, but big ones include:
1) Mistaken belief system to unravel
2) Temporary Injury Issues
3) Temporary mental issues (stress etc.)
4) Exorcising an ingrained habit
5) Working through the gap between having been told what you have to do and understanding what you have to do
etc. Sometimes, when you're dealing with people who have never learned a skill that has plateaued like this (or it did but they've got rose-tinted memories about it), the frustration can be enough to quit trying to fix the issue, and what could be a blockage for a few weeks becomes one that lasts for months or years for no good reason. It's true, though, that the stress of this is enough for some people to say 'look, I have enough fun in my skiing - I don't need this'.
The important thing with this kind of plateau is you have to lean in to it as hard as you can because the pay off will probably be explosive when it eventually arrives. And if you hold on to that belief and attitude, it'll also minimise your stress about not making short-term progress.
This video is worth watching as one way of thinking about the process learning physical skills.
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On the other hand, there do seem to be people in our industry hitting firmer thresholds. For most of our guests it is 'I can make it down all of the terrain types I care to ski safely' or 'I can keep up with the kids well enough'. I think that's reasonable given the time investment they are making in to the sport and I don't want to dig too much deeper in to it, except to say: we have to keep giving them something to aspire to. That means that once they can ski everything, they need to care more about trying to improve the how. 'I want to look like that coming down', etc.
For people in the instruction business, I think the most common place to top out is at the PSIA L2 level.
I believe the L2 standard is attainable to essentially anyone who is mentally and physically capable of holding down a ski instruction job as long as they are willing to make at least some preparation. It's attainable for anyone with one season under their belt if they have a strong pre-existing skillset in the sport and/or are willing to make a continuous and dedicated training effort.
The L3 level (at least in RM) tends to hold people up more, and the why of that is straightforward - skiing sodbuster or prima/pronto in the fall line is an athletically demanding activity. That doesn't mean you need to be in prime athletic shape to do it - you don't - but you lack athleticism you are going to have to make up for it with stronger technique so that you can make consistent, fall-line turns in big bumps in 35->40 degree double black bump terrain without making major errors, probably on a bad snow day. That is where the majority of candidates in PSIA-RM get stuck.
They get stuck on other things too, but that seems to suck people in the most. Why? Well, a lot of it is things mentioned really in the first part of this post, particularly the first and last points: a lot of people have a lot invested in belief systems about their skiing that aren't really accurate, and that makes them poor listeners: they hear feedback, but they don't really hear it.
Having trained people at all these levels and having done all of this myself recently, here's what I see as the biggest underlying belief system that causes the problem:
Everyone in this business thinks they're awesome and attaches a lot of ego to the way they ski. And that's cool, and your god-given right as a ski instructor. But the effect of that is that it makes pros in our industry believe they only have to make a 'few small tweaks' or 'a slight adjustment' to attain success. No. You need to tear out a whole section of your book and rewrite it. You can't tweak your movement pattern, you need to create a brand new one. Build an entirely new initiation to your turn, or an entirely new finish. Start at first principles again.
To paraphrase someone else: it's all about embracing that you suck, and trying to suck at a higher level. That's what elite skiing is built on, imo.