When these people are strong and healthy they need to foster the next generations so that the party has a good idea who their successor should be so that there party doesn't fall into shambles and infighting every time one of them dies or retires.

The problem is that they don't do that. It is in their interest that there are no viable alternatives to them because good alternatives to them threaten their power. Until they are suddenly 80 that is and nobody knows how to replace them so they have to sit on their thrones and hold on for as long as possible.