First past the post is a permanent arrangement

We cannot get rid of first past the post by argument. If we assume one of the parties is to the right (the Tories) and the other to the left (Labour). It is the party of the right which is most susceptible to arguments concerning the nature of democracy. If it can be shown that pr reduces the size of government then it is difficult for the right to reject the proposal. But the problem is that arguments in favour of pr that show it will reduce the size of the state are also arguments that show being socialist is a bad strategy (for parties) and that voters prefer freedom. If voters prefer freedom then there is no particular problem with fptp because voters will reject socialism. If the only reason to reject supporting the Tories is their support for fptp then you have little to complain about. If all of their other policies are to your liking then it seems absurd to complain about such a narrow issue. Tories will not be satisfied with arguments in favour of pr because they see no reason not to vote for their own party (by definition) and it cannot be true that only fptp is preventing you from voting for them. If they were compatible in every other area you would not hesitate to support them. So arguments for pr do not work against the Tories and Labour too have little reason to change their position and support pr. They do well from people voting tactically against the Tories and if it is true that fptp favours socialism they will be happy to 'punish' liberals by retaining fptp. Labour have no interest in removing fptp and substantive arguments in favour of pr are anti-state and anti socialism arguments which we would not expect Labour to be amenable to. Labour are happy to retain fptp and punish the anti-socialist right and so we have an impasse. Fptp remains because it is not in the interests of either side to remove it... either rhetorically or tactically.