Our PM is sure they will if we threaten no-deal for long enough. But putting that to one side, if the UK said ok this has been a fuck up from start to finish but politically we need to go though with it but we don't want to burn all bridges and do not want to threaten the break up of the UK. So let's scrub out the red-lines, we are happy to stay in the CU(or as near that makes no difference) and we are happy to keep FoM(just change the name). What is wrong with that?
the obvious answer is, that would not be a 'real' Brexit and round and round we go. So, they probably won't renegotiate as what we currently have is a UK problem. A problem that, at least as things stand, is unsolvable because we cannot get a better deal than being a member. As a result, there are not enough people prepared to accept that we will have a lower standing in the world. Until that reality is accepted, we are not going to get anywhere as a country