My 94 RS Oilhead is now at 172k on the original untouched final drive. I change the final drive Castrol Full-Syn 75w90 fluid about every 20k and check for any play monthly. I don't sense any thing about it not making 200k+ on the original FD.
I'm now in the process of building up my 07 R12RT into my version of a R12RS. I bought the cosmetic crash/clear title RT with only 5900 miles on it, fully rideable, at a great price. I hope to have good luck with the FD on this one, but I'll be watching it closely. Perhaps I'll do the fluid changes much more frequently than I did on my 94RS.
I have heard though that many failed FDs are due to seal issues allowing water/contaminates into the bearing, or water gets in due to overactive cleaning by owners with pressure washers?