The issue of restoration is an ethical, not moral, problem, and, as such, is not one of right and wrong.
Furthermore, it is a complex issue, found in many collecting areas, not just in bikes. The approach to restoration may depend on cultural, financial and aesthetic considerations.
To begin with there is no test for the importance of a "classic" bike, let alone for its condition. The only principal (even this is somewhat arbitrary)I can apply is what I like to call "duration of esteem." Has the bike been around and appreciated widely for at least 25 years? If the answer is yes, I think it's fair to consider it a "classic". Note that the yardstick for other antiques is 100 years-a much better test. The reason is that after a century any form of favoritism not related to the bike itself(or other collectible) will have waned, e.g. its having been ridden by a famous racer, etc.
No dogma exists regarding the restoration of a "classic" bike. As far as other collectibles are concerned, restoration depends on the condition of the object itself, its value and the aesthetics of the culture of the owner. Art works (fine art) must frequently be restored to prevent further deterioration. In Europe antique furniture is quite frequently restored to "as new" condition, however, in the US a "patina" ,no matter how scabrous, is preferred. I take the same approach with classic bikes. If the bike is deteriorating, I would prefer to restore it so as to prevent further damage. Of course the amount I'm willing to invest will depend on the value of what's being restored. The less I have to do the better-and the cheaper too.
The issue of aesthetics is impossible to address on a purely objective basis and is often paradoxical. The condition of a bike depends on how well time has treated the bike-it's often a question of chance cooperating with time. If someone feels that the scars of longevity add "character" to a bike, he's likely to leave it as is. However, in American society blemishes of any kind, let alone those produced by age, are little appreciated and are likely to be corrected-whatever the cost.