I have a feeling old fruit trees fail suddenly. They seem OK, and fruit well, then you notice more disease, more infestations, a bit less vigourous. We once had quite a large old orchard for several years and found this to be the case. Sometimes an old tree can be rotten in its core and still fruit well. Other idea . Any ground work been done near the tree? We had that as well when we made a patio and a garage base near an old apple tree that was fruiting well. It lasted a while longer then needed taking down.