EU Citizens' Rights Post Brexit

The EU has rebuffed the Prime Minister’s generous offer about the residency rights of EU citizens after we leave the EU.  Michel Barnier has demanded exactly the same rights for European citizens as under existing free movement rules.  Presumably Barnier feels that once a citizen has acquired rights then they should retain them and that EU citizens living in the UK after Brexit should be subject to exactly the same rights as any other EU citizen living in any other EU country.  He does not seem to grasp that post Brexit UK will be a sovereign country and not subject to free movement rules (or the ECJ). So, two snags strike me.  Firstly, what if, post Brexit, EU citizens’ rights change?  For example, suppose, in the future, EU citizens acquire rights about euthanasia that are not lawful in the UK.  Is UK expected to grant those new rights to those former EU citizens that were assimilated during Brexit?  Secondly, how will it be possible to govern our country wherein a substantial minority are subject to a different code of law?  There is a simple solution to this impasse.  EU citizens living in UK should either accept Mrs May’s pragmatic offer or return from whence they came – there are plenty, around the world, who would be happy to take their place.