There is that risk, or you employ a good manager and he gets poached by a bigger club. But just like you should be scouting replacement players, you should have in mind replacement managers/coaches. A list of people who play the same kind of football as the incumbent, who you could attract.
I think you give him too much credit. If he saw anything in playing Joelinton up front, on his own, for much of last season it was lost on me, the Newcastle fans, the pundits, the journalists and the wider public.
Almiron is a number 10, but he's been played as a winger, a central midfielder and as a forward. Hendrick is a 2x4 plank, but has been played as an actual footballer...
Bruce is out of touch, out of luck and hopefully out of a job soon.