Many changes can be reversed (saddles, 700c rims, etc) in bicycling, often within a few hours time, so the consequences of changing parts isn't as great, provided you keep the original parts. It seems to me that classsic bikes from the 70s and 80s fall into two broad categories:
1. Original-spec or period-correct bikes that were expensive and fancy and rare even in their day. There are relatively few of these.
2. Very nice classic bikes that may or may not have updated parts. Ranges from stuff that in its day was top-of-the-line (but not rare of custom) to stuff that was the everyday person's bike (and still is). Seems like most of us on this forum have these.

Similiar to vehicles, an original, pristine classic is only really valuable if it was valuable in it's day, too. A pristine Corvette is worth collecting, a pristine K-car, not as much. Still, like vehicles an art, buty it because you want it, not as an investment!

Updating components will not add $ value to a classic bike, only riding enjoyment value. If you upgrade/alter a bike to increase riding enjoyment, this increase in enjoyment will easily surpass any lost dollar value in 99.9% of the cases. And as I said earlier, most of the changes you would make can be reversed.

I'm sure I'm missing some subtleties here, expect others to chime in soon.