They can hire a mid/ senior developer who will get up to speed fairly quickly and be able to work on their ownand then will leave after a year or two, or they c

They can hire a mid/ senior developer who will get up to speed fairly quickly and be able to work on their ownand then will leave after a year or two, or they can hire a junior Dev who will take longer to get up to speed, and use up the time of other more senior devs, or increase tech debt if they go off on their own, and then they'll leave after a year or two. To put it another way they can either pay a senior developer a lot of money to do work for them, or pay a senior developer a lot of money to teach a junior Dev (who they're also paying),only to lose them both after x amount of time.