Of all the "restoration" that has been done on these guns as a whole, my estimation is that most has not been done well.
Ask yourself whether restoration as a whole has been largely succesful or has it caused a lot of problems, messed a lot of guns up and resulted in a great deal of history to be lost.
Jim
There is a good deal of truth in Jim's comment, but it is good to remember that restoration work has been going on for a very long time now. Dillin comments on it in his book, and he wrote the book in 1920ish. Kindig mentions restoration, and supplying the brass for a barrel stretching on a well known gun.
I believe it safe to say that most, if not all the work done years and years ago is no longer up to today’s standards. Plus there are the guys that have done work then, and now, when they shouldn't have. I’m sure guys have looked at restored guns and asked themselves what in the world was the guy thinking, just like I've sometimes looked at a new build gun and wondered why the guy ruined a nice piece of wood to make that thing.
As for destroying history; In the long run, I think not much. In the first place very very few of these guns come down through time with any history attached. In most cases you’ll be lucky to learn the name of the previous owner. Generally speaking, if the gun had any history, it was long ago lost. Some might consider cut back barrels, percussion-ized flintlocks and fullstocks made into halfstocks the guns history, and in a perverse way it is, although that history is a series of detracting modifications done to it taking it away from the way the maker made it.
Plus, most all had a very hard life once they were past their usefulness. Many were just tossed in the trash, given up for metal drives during WW1, given to kids to play with, etc. Those that remain have been beat and abused, broken through the wrist, have had the forearm busted off, had the silver inlays pried out and sold off, and even fewer remain in original as made condition with full length barrels, stocks, and lock ignition.
Chances are with a lot of the guns, if some restoration guy hadn’t done his work on it years ago, a lot more guns would have been scraped, trashed, abused, and completely lost to us nowadays all together.
On the other hand, I applaud the guy that refuses to buy any gun that is not totally original, or refuses to own a Model T Ford or a 58 Corvette, or a great masters painting that is not totally and completely original. I’d love to see their collection, small as it might be.
John