I think there is a lot of assumptions being made in regards to what training actually is. In software development field change is constant a senior dev can not afford to stop learning new techniques, frameworks, etc. that’s not something you just get mentored at and you’re set for the rest of your career. The issue that seems to be ignored here is the fact that good devs are active learners on their own and do not typically require the kind of hand holding that is implicitly implied to be required in these type of arguments. A company can not train an employee who otherwise does not have the required skills for the job and hope that once they are trained will stay at the same company.

Here is the equation in my mind:

cost of junior dev added slowness + senior dev lost training time > recruiting cost for senior devs + lost institutional knowledge + ramp up time for new senior devs