It is common knowledge that the average developer leaves after the 2 year mark. When that happens, everyone else is left holding the bag and they become the new experts.
Companies are full of self created abandonware.
Another person leaves and it is now two extra things that the vet has to become the expert on. Then three, then four and on and on. Next thing you know, your vet isn't getting any work done because they are holding too many bags. As the expert, they are in too many meetings trying to keep other people productive. They are pulled into too many critical issues and they are trying to mentor new developers in your ever revolving door of new people.
But they are unproductive
No. They are keeping your company afloat and you aren't compensating them for it.