One other issue in retouching is the color changes that paints naturally exhibit over the years. Paints from the 1970's are especially unstable, as anyone with a Raleigh Team Pro knows about. Those bikes started out red, but by now, almost all the original finishes are orange and are still changing color.
This is not a Raleigh/Ilkeston-specific problem, it affects ALL paints of the on-topic CR era. My roommate in college did a GM-funded undergrad research project in the EE department of MIT to predict paint color changes, because car makers of the mid-80's were forced to CHANGE touchup paint colors EVERY YEAR to match the never-constant paint colors of older cars. So a 1996 altima paint retouching can has a different formula in 2000, 2005, and 2010.
Anyway, even with a perfectly invisible touchup, in a few years you could expect to see a bicycle filled with leopard spots, especially if it gets any UV exposure, as the 1970's paints, which were less stable, will change color and fade faster than the modern paint retouching colors.
THis is just another reason to prefer a full repaint with accurate decals vs. a touchup of original paint, if the bike has no historic value.
I agree however, that emotionally its hard to lose part of your old friend, i.e. the original paintjob on your original bike.
- Don Gillies
San Diego, CA, USA