You are assuming the employees will just accept their exceptional situation instead of using it to gain some time and jumping ship. You are also assuming the managers know beforehand who are high-value employees.
Yeah, I am. To be fair, I inherited that assumption from the GP.
Anyway, you are right, mature business that have no further space to grow may not want to keep them at all. Even when they aren't a differential, they are still worth more than they cost on maintenance, but there are plenty of companies that just skip maintenance and it's not obvious at all if they are any worse for it.
I doubt either of those are true.