labrat, i definitely see what you are saying, and i think u answered your own question. i think u have to look at youw own family history as a guide and not look at anyone else's. the range of outcomes other people will tell u they experienced will be too disperse to tell with any certainty what will happen with u. u have your own genetic pattern.
with that said i hd a grandpa who was much the same way as yours, so maybe i'll be they same way as him.
if i were you, i wouldnt be too worried about progressing to much more. your family history and response to proscar are good indicators for this. of course im not a ht doc so i would consult with them, but no sense in going thru life (which statistically is half over for u) worried too much about this. even if u waited another 20 yrs and got a ht at 60 what would that accomplish? u would just worry if u would advance over the next 20 yrs.
most likely, your donor hair will stay strong well into your 60's and likely even 70's. u have already shown proscar works well for u and it will likely continue to work well for u. just my two cents..