Such a sad ending to a long life, but if he was declining, perhaps his humans did right by him. They did what they felt was the right thing - at least they didn't let him suffer needlessly, as I'm sure they wouldn't have had the drive that would have been needed to properly care for him.

RIP Puss. You are now free and healthy once again. I wish we could have gotten to know you better, and under happier conditions.