Alternatively, when your outsourcing agency finds they have accidentally assigned you an actually good engineer, term used loosely, that you're not paying for (they know this happens when the engineer gets up to speed with your codebase at all), "your guy" is replaced with another guy who inherits the name, the email address and the SSH key.

And if the agency doesn't do that, the good engineer will figure out he's being underpaid as slop-for-hire cannon fodder and move on his own accord.