I agree that it shouldn’t sound like a threat, but there is no team. He’s the only person. There is only one way for the employer to feel the pain.
I've got one person under me and have done for 3-5 years.
Most have enjoyed working under me (so I understand it), most resignations have been for bigger opportunities / bigger cities / bigger money (1 out of 4, and TBH I missed they wanted more money - this was my fault).
My current "report" seems very happy (though I expect them to leave soon, not for money but they are far more interested in another industry and tech challenges that come with it).
They could do infrastructure, kind off, not code - no chance (I like them as a person but they depend on me to teach them how to code)
Hire "younglings" (star-wars) is my remit.