Even if your junior-turned-okish developer goes away, if you did a good job and part on good terms you now have a working advertisement in another company. And

Even if your junior-turned-okish developer goes away, if you did a good job and part on good terms you now have a working advertisement in another company. And maybe a couple years down the line they'll come back. Investing in your workforce for the long term: when you're big enough I think you should do it. Having a "reserve" is also useful. You don't have devs doing nothing: you have devs learning, teaching and ready in case a business opportunity appears.