Answer: According to Stephen Page and Joseph Smillie, authors of The Orchard Almanac, old, neglected trees can be difficult to rejuvenate. It depends on how "far gone" they are, of course. To judge the trees' health, look at the growth they make in a single year. If new branches grow 12-16" in a season, it's probably worth the effort, especially if the trees have other attributes you value, such as their inherent beauty and place in the landscape. It sounds like the trees are productive, too.
Orchardists thin fruit on their trees so more energy is focused on fewer fruit, which increases size and quality. As you've seen from their symptoms, apples and pears are succeptible to many diseases and insect pests. It may take a few years for you to "clean up" the reservoir of pests and disease spores that exist in the immediate area of the trees.
I recommend that you page through a copy of The Orchard Almanac, and see if it makes more sense for you to plant new trees or attempt to restore the current ones. Hope this helps!
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