I think the concern is that they might become good enough for a senior to not need a novice. At that point where to the seniors come from?

We’re here already. Nobody wants to hire junior devs anymore. Seniors are still a couple phone calls away from new positions.

I got my first job in 2001, it was like that then as well (maybe even worse). It got better when the market picked up again. I’m confident there still won’t be “enough developers” when then happens just like there’s never “enough electricity” despite the fact the power grid keep getting expanded all the time - people fine a use for even more.

If developers become more productive (both juniors and seniors), there will be more demand for all of them.

Or few will be productive enough to cover everything. Compare amount of bobcat operators vs ditch diggers.

There's an almost unlimited amount of demand for people who can write software or automate things, if you can lower buck-per-bang enough.

That "need" is not as a helper but a cheap way to train the next gen senior which by the time they are senior know the ins and outs of your company's tech so well that they will be hard to replace by an external hire.

If your approach to juniors is that they are just cheap labour monkeys designed to churn out braindead crap and boost the ego of your seniors then you have a management problem and I'm glad I'm working somewhere else.