Full disclosure, I'm not a senior developer, but a senior elsewhere in IT facing this exact same situation.
Like the author, I've also identified our cannibalism of our juniors with the obvious future consequences to this if continued unabated. I've struggled with how to combat this, and I really give the author lots of credit for producing actual actionable plans on a path forward.
The main failure mode I see for the proposal is organizational self interest. Investing in juniors made sense in the past because the junior work (grunt work) was needed completion anyway so the exercise of training produced a useful result while also freeing up cycles for seniors to engage in higher level work. The result is that the training of the junior became partially subsidized by their output. Today that isn't really the case. A senior with an agentic workflow can likely produce the same result as the junior with only a rounding error of effort. This shifts the organizational incentive model significantly to the point were training the junior is purely a cost sink with only a negligible return to the organization.
The author points out properly that systemically across the industry cannibalizing juniors eventually means cannibalizing future seniors, but wrongly assumes that training juniors inside the organization means producing seniors long term in the organization. Unless the entire industry adopts the new system to train juniors there will be organizations that simply poach juniors that have been trained up. Money is fungible. If it takes money to train the junior an org could take those same dollars and offer relatively obscene salaries to trained juniors.
From my view, seems like it will always be more expensive to train than to hire those trained. What am I missing here? How can an org that trains up its juniors (using the author's good suggestions on how) guarantee they keep their trained juniors (or at least retain enough to make the whole exercise worth it)?