In fear of stating the obvious shooting a shotgun does not wear the barrels out. The main culprits for barrel demise is firstly poor maintenance not every owner cleans his gun after each days use. Secondly some of the more mature guns where used with black powder and the material used in the primers was as aggressive as black powder residue. Then there is the old enemy RUST!!!! Caused by the first three!!! But the largest cause of barrel wear is the gunsmith with his attempts to remove the rust and pitting by lets just say reaming out the bore and in doing so making it larger.
Now here in Brit land as I am sure you know there are finite limits on bore size you just cant ream out the bore to remove all traces of pitting go too far the gun is valueless and will require a re-proof with all the risk that entails to restore its value, but NOT on your side of the pond export it from here and its value returns and this happened a lot in the past. You can do more or less whatever it takes to end up with a mirror finish bore. And may be that puts in perspective why there are a lot of guns with oversized out of Brit proof bores on your side of the water that will command a high price but over here are just bad news to the owner. So lets level the playing field if your Parkers had to live over here with the same rules of proof a Maritime climate over the same time scale you would see a lot with new sets of barrels.
Though with correct and regular owner maintenance plus a trip now and again to a reputable gun smith, a lot of these finely balanced marvels of true English gun making are still in continuous use after some one hundred and fifty years and still comply with the Brit proof laws.
So in the end you folks are extremely lucky that soundness of a gun is what is correct in your opinion you do not have the government deciding it for you!!!!!!!

The only lessons in my life I truly did learn from where the ones I paid for!