The plumber is responsible for the install of the product. That said the best thing, since the job is done, would be to come to an agreed price for compensation and you live with it. BUT If you realy want it replaced, and you are intitled to that, They should replace it or refund the replacement cost. Yes they are leagly liable but court is the last thing you want to do. It is a PITB. Have you checked with companys that repair scratches like that? They are expensive but that may be the way to go for everyone. The plumber is responsible unless he told you other wize before the job was started.