The UK hasn't left. The UK and EU haven't agreed anything yet. When the deal is done (or there is no deal), judge it on its merits then, and only then.
For the time being it is a case of the EU / Remainiacs harping on about how Britain has fucked itself, how everything will be a disaster and how the EU needs to set an example to anyone else who might think of daring to leave, whilst the UK attempts to hammer out a deal with people who really aren't interested.
If May had any guts, she'd ask the EU for the right to offer Britain a referendum on the final deal - a choice leave with deal, leave without deal, or stay after all. In my view it would be difficult for the EU to justify not allowing this, especially if the EU really needs Britain's money as much as everyone thinks.
Yes it would piss off Nigel Farage et al, yes it would piss off a lot of Tory voters and probably end her career, but equally it might at least win her some respect and lead to a cushy job in the EU machine.
If the UK voted leave again, the matter would be settled beyond all doubt. If it voted to stay, well, that's democracy, isn't it?
In the meantime, Remainers should try to keep their arguments polite and factual. Telling a Leaver that they've fucked up, that they're idiots, that they've wrecked Britain, will not make the Leaver change their mind. The EU punishing Britain will not make the Leaver change their mind. It's this feeling of being told they're wrong and unimportant that pushed many people into voting Leave in the first place.
The referendum on leaving the EU shouldn't have been necessary. The Leave majority and the feelings that caused it certainly weren't necessary. But Remainers should not deny the feelings that caused people to vote Leave - they should have, should still, try to convince Leavers of the benefits of membership.
It shouldn't have been a question, but it was, and it has been answered. For now.