I'm in my late 50's. I try not to live my life worrying about how much time I have left. Who knows what will happen in 10, 15, 20, 25 years? My own view is, keep adopting, do your absolute best to give them the best life possible, and let the cards fall as they may. Maybe, probably, as you said, that one would have died. If you can only give it 6 good months, is that not better than letting it die miserably, sad and alone, right now? Maybe a kitten you save gets leukemia (hopefully not) and the year you gave it when you were 94 years old was a wonderful year for you both, and maybe the last year either of you had?
Not at all trying to invalidate your question, because it's an important one. I just think there are too many variables to consider, and each person can only do their best, and hope it's good enough. And I'm glad you adopted the kitten.