My thinking is that a nice bike "back in the day", an active rider would have very likely made upgrades and changes. Some for fit, some because parts wear out, some for better performance. I know I did this myself with my college UO8 back in 1975. I changed out the rims to alloy, ditto seat post, then I rounded up a three piece alloy crankset to replace the cottered crank and so on. Rode the heck out of that bike.
As far as bikes I pass on, my
#1 priority on a keeper is the frame cosmetics: paint/decals/rust. Even then, my 1988 Cimarron LE violated all those rules. So sometimes I waver. I really could care less about components as far as originality, except for on a collector bike like my 1973 Schwinn Paramount. None of my other bikes have original components, all have been upgraded. My Prologue came to me with blah Shimano 105. No thanks.....
I've got three sets of those Mongoose Pro Class wheels right now.....