BGHNTR416 - that's correct. If the barrels are 'off face', it is a relatively easy matter to sort that out (by cold-welding additional metal to the lumps).
More serious is the erosion of the barrels by firing x-number of rounds through them. In the UK it is illegal to sell a weapon that is out of proof, with the result that a lot of shotguns on the verge of failing the minimum barrel thickness recommendations come up for sale, as well as shotguns that have - or would - fail re-proofing, so are sold as lock and stock only.
The cost of rebarrelling a shotgun is typically about the worth of a second-hand shotgun: ie. you have to buy the thing with worn out barrels, buy some new barrels, and you are left with something the value of which is not much far off from what you paid for it, but excluding what you spent on it.
Some 10 years ago, Holland and Holland quoted me £12,000 to rebarrel a shotgun (which was about what the shotgun was worth). About a year ago, Rigby quoted me something like £25,000 - memory fails me slightly - for rebarrelling (which was considerably more than the shotgun was worth). Westley Richards won't do it at all. If you have a pair, you run the risk that the barrels will be heavier or different between the two shotguns (even if they went back to the original maker - I have a pair of Westley Richards like this).
A shotgun is, like any machine, subject to wear and tear. At some point it becomes worn out; unless you have some very good reason to spend a lot of money on it - which you will never get back - it is best to treat it as a decorative object and buy a new one. Alternatively, you could run the risk that it will, literally, blow up in your hands.