A medical breakthrough will vastly increase the lives of our pets. Animals who previously lived only a decade or two will live many times that, with cats and dogs living to 60 or 70 years. This will make buying a pet as an adult a difficult decision, because you must consider what is to become of your animal companion if you die before they do.
Some animals will live many times beyond that -- turtles will live to 500 or more years, and will be more like family heirlooms than pets, passed from one generation to the next. Our pets will no longer merely be our best friends; they will be our lifelong friends.