Over the years I've heard stories (oral stories) of ML barrels that were "shot out" and had to be "refreshed". I think you have to take a lot of things into consideration . . especially when talking about originals. The steel used for the barrels varied . . we know that for sure. Also, remember that they were "utility tools" . . i.e. they were shot, often reloaded, kept loaded, stood in the corner, etc. so that the fouling had more chance to "work" on a barrel. Lubrications varied. My grandfather (born 1867) had a 45 cal. half stock "plains rifle" that he had purchased from one of the pioneers who came into Michigan in the 1830s. He shot it for years. His patch lube was bacon grease. Bacon that was salted . . which in turn, would work on the barrel.
Riflings varied as well . . some shallow, some deep . . . many "hand cut" on a primate rifling machine as opposed to machine rifling. My point . . . I suppose that a barrel with shallow rifling, when considering fouling, wear, etc. could be "shot out" over time. In looking at originals, we've all seen a variety of rifling . . . some even resembling a hexagon (six lands and grooves) while others, round.
I've seen, handled and even repaired a number of originals . . . half stocks, full stocks, etc. The one thing that I have noticed over the years is often times there is "muzzle wear" due to large numbers of rounds being pushed home with wood and steel ramrods, etc. That's just the "nature of the animal". No matter how you try to keep a ramrod clean, it is exposed to lubricants which in turn attract "dirt".
I have also worked on originals in which it was very evident that they had been "re-breeched" at some time . . . i.e. breech cut off, rethreaded for breech plug, etc. Pin lug locations changed, etc. I have also seen and owned original barrels by themselves in which these changes are evident.
The majority of us today, utilize our ML rifles/pistols as something that is taken out for target practice, shoots, etc. . . . and yes, hunting . . . but my point is that we probably clean our weapons much more often and much more thoroughly than our ancestors did. We "prize" them . . . and there's nothing wrong with that. As a result, there isn't as much fouling, etc. left in the bore to "work" on the rifling and cause damage. Just think of how you clean your Colt or Remington . . . a far cry of what was actually done in the field when they were often submerged to the grip in hot water and lye soap.
As already referred to . . . look at the 22. I have some that are 60 years old that have had thousands of rounds put through them - the bores are still pristine. On the other hand, I have my Dad's old 1915 Stevens Favorite that also has had thousands of round put through it. The rifling on that is still very evident but I would rate the bore as only fair due to the pitting, etc. caused by a lack of good cleaning and fouling from early cartridges which took its toll over the years.
Personally, I think that the barrels today, whether they be from run of the mill "kit"guns or such manufacturers as Green River, Green Mountain, CVA and even older barrels by such folks as Bill Large will last a person a lifetime if wel taken care of. If you're like me, you have more than one rifle/pistol, etc. that you shoot so you switch off. I suppose if your are a person who "shoots one gun", . . and you shoot it everyday, there would be a possibility of "shooting out the barrel" . . . but I would think that would be the rare exception.
Just as a matter of curiosity, I'd like to hear from those who strictly shoot ML shotgun. I have a brother who shoots a lot of sporting clays, etc. with his ML double barrel. He goes through a lot of rounds every year but I haven't ever heard him ever say anything about internal barrel wear. Yes, it's a smoothbore but one barrel is choked so I would have to think that at some point, with all the shot, wads, etc. leaving the muzzle that there would be wear to the I.D. of the muzzle.
Just my worthless thoughts!
I mean seems to me it would take a BAZILLION, seeing as how the patch is nice and lubed up and we're not shooting jacketed bullets with a steel core. Should I put the old barrel (the barrel is in good shape just the blue is worn, the bore is fine) back on my GPR and save the new barrel for a later? I do think I will put the old barrel on just to beat around in the woods.
. . . . Dangitt, I'm done too old to find out how long it'll last! 
lof this stuff