Which, as far as I can see, has led to a vicious circle where developers will often refuse/bitch/complain about working on something that doesn't increase their immediate employability prospects. Not to mention CVs that quite often have a half a page or so of solid acronyms to increase the chances of being found by simplistic recruiting company search engines.

I agree. I think the logic is: "if we train them, they'll go find better paying jobs, and besides, as soon as this project is done, everyone will be laid off anyway." So everyone only wants to hire exact fit.

My experience differs - I think it was in fact the rule of thumb in the 90s... until the dotcom gold rush got sufficiently out of hand that hiring people and training them became the only reasonable solution.