With a slight change, it would no longer be possible to shoot antique revolvers off ranges, without paper.
Folks are paying a premium for these exempt pistols. If that special status is lost, the value of the gun will be based only on its collectability, not its hassle free shootability.
For a specific example, consider a Mk. I Webley in .455 vs one altered to .45ACP. If they cannot be taken out and shot without a permit, which do you think would have more collector value? Altered or unaltered?
Then there are the clunk revolvers - cheap guns when made, doggy condition now - but they sell for high prices because they are paper free shooters.