Collectors are all about originality. If they just wanted something that "looks" original, they could buy replica guns rather than searching out the real thing. They would save a lot of money and time that way.
I think the comparision to cars is not as valid as, say, a comparison to antique furniture. How many times have you heard this on the Antiques Roadshow --
"If you had not cleaned this cabinet, it would be worth $200,000. But since you did, it is now worth about $30,000."
And that's just cleaning it! Not a restoration! At least a careful cleaning will not ruin a gun's value.