Well it's also a pool, meaning it rotates, you get some, lose some, get some, lose some, etc. Maybe a dev will take 2-3 jobs to learn the trade (PHP here, Js there, some fundamental web stuff to top it off, and here's your "professional-grade developer". Great.) But you get to hire equivalent devs at each step, then it's just about $/skill.
Now, if all companies made it part of their "offer" to train people "enough" (say, 1d/w), then you'd expect all the workforce to become more qualified, better in time and in age.
You could actually pay/recover the "investment" of training the equivalent of university/grad/postgrad/etc for all employees simply by the fact that everyone else would do it too (and it would certainly lower wages a bit for the early years of these newcomers, since they'd skip the idling 20's decade of many youths currently).
I don't know, it's clearly not something you could do overnight or even over a generation, it's likely to be deeper and more 'revolutionary' than that in people's minds; but mathematically, economically, it tends to make sense (we've done that for years with "guilds" and "companions" in the medieval ages and actually since forever in some trades).
I think the current mainstream / massive education (take hundreds, thousands, and grad them each year) is just the result / need of industrialization (requiring an educated workforce), a novelty of the late 19th and 20th century.
I think the cursor is moving and the explosion of alternative means and times/ages of learning is a strong indicator of that.