So where do these seniors come from?

That’s beyond the planning horizon.

You could have a much smaller pool of juniors/journeymen that are focused on maximum learning rather than amount of output.

without juniors, how do you get more seniors?

Maybe programmers will adopt a similar model to artists and musicians. Do a lot of work for free/practically nothing hoping that some day you can make it "big" and land a full time role.

That is already the recommendation. Pick an open source project and contribute.

And most people who consider the job are driven away by the economics of it.

we already have that with unpaid interns and PhD students