Playing devil's advocate here, but if this is the expectation, then it perfectly explains why the problem exists. If the company pays the training cost AND ups

Playing devil's advocate here, but if this is the expectation, then it perfectly explains why the problem exists. If the company pays the training cost AND ups the salary consistently for the juniors, then why would they ever hire and train them? They can just hire them after they've been trained elsewhere, pay the higher salaries, but avoid any training and development costs. This is exactly what happens in the majority of companies, and they only hire seniors. Which is what prompted the OP to ask the question in the first place.