Senior engineers have always owned the application (by knowledge not by law). And that has never been a problem. The real problem is, that without new Juniors there will never be new Seniors, and your company will collapse when your Seniors retire.
If this was a "your company" problem, it would sort itself out soon enough. The real concern is that it might a "your industry" problem.
I have seen this happen in some companies already. A slightly different scenario, the CEO actually wants to hire new juniors but seniors refuse to train them .
It happens cyclically. Sun Microsystems was a good example, Cerner was another (ironically both acquired by Oracle).
From another perspective, the problem is people entering the workforce without sufficient skills to be valuable to employers anymore. The solution would be to spend more time in training so you can reach senior level when you start. Software has been an anomaly among high-paying professions in the low bar for entry. Maybe that's ending.
The hypothesis is completely wrong. With AI, juniors can catch up with the codebase / domain must faster and deeper than before. It's just a matter of putting time.