First past the post and pr are very different

Only if fptp and pr are the same would it be consistent to defend fptp. If it makes no difference then we might as well keep the system we have grown accustomed to. But if there is a difference and we know that government is constrained by democracy then we would want to be sure of having the best possible type of democracy for a good life. Since there is little argument that fptp is not the best system (democracy is about choice) then to establish an argument in favour of reform all we need to do is to establish that there is a difference. Once this is established and shown then the motivation for reform becomes obvious.