Back to the OP's initial question... what is the terminal intermediate plateau?
Here in the North East, it's a common way of skiing groomers that prohibits further progress. Doing these turns "better" does not improve control over line nor speed on hard snow groomers, and does not help the skier do bumps or trees with increasing control. In spring when the snow is soft and gloppy, it doesn't work.
Why is this way of making turns terminal? Because adding to this movement pattern, or doing it "better," does not produce progress. It's a dead end. Fixing it means replacing it. Replacing a habit is like going on a diet. It's hard and unpleasant to do, for many skiers.
How did this happen? Well, some essential fundamentals got missed as the skier moved from beginner to novice to intermediate and moved to skiing blues and blacks regularly with friends. Things seemed to be working OK when conditions are good, so repetition embedded the movements deeply into muscle memory.
Those missing fundamental movements need to be learned if the skier is ever to get off the plateau, but there are already habitual movements in place that seem to be working. On a good day, when the conditions are just right, the current turns feel great. It's really hard to replace those habitual movements. The skier needs to go back down onto low pitch terrain and do the new movement pattern with the missing fundamentals so often that it starts getting embedded, then slowly take it up the mountain onto green then blue terrain. If the skier continues to "ski just for fun" on normal terrain daily, with only a bit of practice with the new movements, that just makes the old movements harder to overwrite. People don't want to do drills and ski slowly on beginner terrain until their bodies get habituated to new movements. It feels awkward, and it's a challenge. They want to ski with their friends. Thus... terminal.
So what are the terminal movements and what should they be instead? It can be different for different people. I'm speaking for New England skiing on hard snow here. I can't speak for skiers out west at big mountains. Any of these in combination can get a skier stuck on the plateau.
--the skier's upper body always points in the direction the skis are going (in shorter turns, the skier needs to allow the skis to turn more than the upper body). This upper body rotation sometimes follows the skis, and sometimes precedes them.
--the skier tilts their whole body as a unit to edge the skis (the skier needs to edge the skis with the legs, not the whole body)
--the skier rotates the skis quickly at the start of the turn so that they point across the hill in the opposite direction (skier needs to be able to get skis on new edges at top of turn at will)
--the skier does not allow their body to cross over the skis at turn entry.
--the skier swings arms with pole plants, or holds arms at the side of the body, or plants poles cosmetically rather than with purpose, or crosses the body with hands with each pole plant (pole baskets should swing, not arms, elbows should be ahead of side-seams on jacket, hands should not cross left-right in front of body, pole plants or touches provide timing sues for turns)
--the skier pushes the new outside ski out to edge it (too many words are needed to explain the better option)
--the skier leans in while pushing that outside leg outward, with shoulders tilted, inside shoulder low, making the outside shoulder to outside foot a long, straight, strong unit. This is called bracing (too many words needed to explain its replacement)
--the skier does not know how to manipulate edge angle with ankles and lower legs, instead using the whole leg and maybe even the upper body.
--the skier does not know how to fully complete turns for speed control
--because of some combination of these missing things, the skier has one habitual turn which simply won't work in all conditions on all terrain. (skier needs to be versatile enough to make short turns, medium turns, long turns, completed turns, incompleted turns, all at will)
There are probably more parts to this, but I've got to get out on snow.