Earlier quoted context omitted.
There is the part where you give junior pay raise after he or she learns enough to get raise elsewhere.
If you're going to end up paying what it would cost you to hire the trained person anyway to avoid having your staff leave for higher wages, that heavily reduces your incentive to put a lot of resources in to training. Training + market rate compensation is more expensive than just market rate compensation.
Also, it seems to ignore the fact that training people is actually an incredibly useful thing to do to hone your skills as a senior dev, and that having to teach forces you to cristallize, simplify and explain thoughts and processes that you possibly never challenged before.
Tbh, I think I'm more productive and learning more when I have a decent intern to coach in my team than otherwise. So it's really a win-win situation.