I think that if we talked about completely original (not even recapped) and still working Amigas, then I think we are talking some time in the not too distant future (i.e. within the life time of many of us). There might be a few original 500s around that still work in 50 years, but I think it will be very few, mainly due to cap failure. I don't think many electrolytic caps will work well after 80 years. But we might never know since most people who care about keeping Amigas around will recap them and otherwise keep them in shape.