So I created a hypothetical situation to illustrate what, to me, is a clear example of a temporary labor shortage. As you say, "the incentive to develop the skills should sort this out" ... and I'd say that's true, but until it gets sorted out, the situation can reasonably be described as a "labor shortage".

Also in the real world the wages are sticky enough that they take time to adjust, and then it takes time for people to realize that X field is becoming well paid and then start pursuing it, so realistically the 5 year period is more like 7 to 10. Plus as long as the industry is growing/expanding, demand can continue to outstrip supply due to that stickiness.

Even rapid moving stuff like AI has bee illustrative of this. The AI leaders are all primarily poaching, and wages are astronomical. This is undoubtedly fueling a massive number of people to study/specialize in AI, but it takes years (maybe even a decade) to complete that path (depending on your starting point).

There is absolutely no scenario in which you could convince me to train as e.g. an underwater welder, no matter how much cash you’re offering.

Because people have to a) take the years to learn/develop those skills, and will only do so if b) they know those jobs are still going to be around for a while after that.