I agree with this and would only add that the process is not linear. Geriatricians are familiar with the fact that people's capacities don't decline gradually with age, but do so in a series of steps - they have a crisis, such as a fall or an acute illness, and although they do recover, they get back only, say, 80% of their full functionality (simplistic to reduce it to a percentage, but it illustrates the point) and then carry on at that lower level for a while, until the next crisis.

In my limiited experience it's the same with the older cyclist. As long as one can continue doing what one was doing, and train consistently, performance degradation is surprisingly small. But when, as in the OP's case, illness or injury intervenes, it is immensely difficult to get back into the same shape and one takes a step down.

Yup. Old people are meant to go gently into that goodnight. It's disconcerting to the young to see us pedalling past them on the climbs.