In the situation with the jr dev, it works for 2-3 years, until they leave because they're not getting raises, and all of a sudden they realize that they don't

In the situation with the jr dev, it works for 2-3 years, until they leave because they're not getting raises, and all of a sudden they realize that they don't know everything (a typical stage in becoming a mid-level) and now have nobody to learn from. Or, when they are getting older and realize they probably aren't long for the company because nobody has their family, values, lifestyle.