First, if your oil pan bolt/threads are stripped and this is the only shop that has touched, then I would say it is pretty much all their responsibility. These things do not “wear out” aluminum or not. They are only improperly handled - like cross threaded and overtightened. You are not at all likely, however, to get the shop to take responsibility. If you look at it from their point of view, they have no way to say that no one else has ever touched the plug. In the end a fight with them is probably not worth your while.
Second, you DO NOT need to have the whole oil pan replaced. There are many many ways to repair this without pulling the pan. If this is the same shop then just get the heck away from there. (This isn’t a corporate chain-type shop is it?). Just get another mechanic who will retap the pan for an oversized plug or something like that. There are multiple ways to fix it.