Might as well chime in here as a few of my bikes fit the discussion.

evidence: 1974 RIH all original (with exception of tires and brake shoes), I prefer it
that way, and prefer the patina that came with the bike.
evidence: 1983 Pogliaghi Ital Corse Pista. Has a mix of components, some older
than the bike (crankset is 151 BCD old), some new (Miche BB) and some period
correct (super record headset).
evidence: 1989 Serotta running Campy Chorus 8 speed with Ergo etc.

None of my bikes are what you would call "collectors", I build them to ride I don't show
them, the are not entries on Concours competitions so for me it's not important.
while I prefer to keep them period correct if I need to mix n match to get them on
the road, so be it.

marty