I don't think major revisions by a studio should happen. However, a studio should work to restore original color, quality and remove damage done to the content by all means. The first time I saw "Vertigo" as remastered, I was amazed at how good it looked in comparison to versions prior to DVD. I think the idea is: how did people originally see it or how was it intended... because that isn't really a revision as more of a correction to the way it was originally intended.
I don't consider fixing print damage or wear as revisionism. However, removing natural artifacts that were part of the original presentation, such as film-grain, is. I'm not making a judgment about these revisions, just asking how far should the studio go?