Because domestic UK politics wouldn't allow this to pass on technicalities, I mean haven't you learned anything from the build up to Brexit? It would have to be explicit to be palatable, not some technicality that we effectively want to play ball on the Euro, but never really do... Hell, even from the EU side I'd want a honest decision, not political illusion.
I'm saying a pinky promise (to join the Euro, never to do so) would not be sufficient to reassure the British establishment or electorate of losing fiscal sovereignty. I don't understand why the EU would want this either. It would have to be an explicit "The UK has full fiscal sovereignty".
