As for a country leaving being worse off that if it were in the EU.
Of course they have to be, otherwise no point exists in being in the EU.
And therein lies the rub. The question is not so much 'Can an EU member do better outwith the EU?' but 'Will the EU allow an existing member state to try?' Don't get me wrong - the UK, IMO, would be better off staying in the EU, but the fact remains that the UK will survive quite well outside the EU too... but if the UK does leave the EU, we'd almost certainly do alot better (and the EU would benefit more too) if a common sense free trade deal between us existed. I guess my main point is that the it actually doesn't make much sense from an EU perspective to go down the route of high tariffs, because the Eurozone has enough (massive and serious) problems without making life harder than it needs to be.
So why did the UK decide to leave the EU without considering the consequences?
Flip the argument on its head and say 'Is the Irish border question a good enough reason for the UK to stay in the EU?'
The fundamental problem of having a hard border in Ireland was resolved by the Good Friday Agreement, but is now back in play because of Brexit. The argument that the UK are responsible for this (because 'The UK voted for Brexit') is only half true - the EU bear responsibility too, for it is the integrity of the Single Market that is the
only reason why a hard border is even being discussed at all.
As I (and many others) have said before, this is an important factor... but, IMO, it is not the most important factor by a
very long way. Peace is more important... the people who live on the island of Ireland are more important... democracy is more important etc. etc. The integrity of the Single Market is relatively
unimportant, and yet that is what takes precedence in the EU's eyes, and it is their fundamental red line.
But I don't believe that it is or should be a question of one or the other - I do believe that it is possible (though not easy) to respect both the integrity of the Single Market (the EU's red line) and the other (more important) factors - by making a legal commitment towards tariff-free trade and an agreement on customs checks away from the border (say at ports across the whole of Ireland) - something a bit more imaginative that forcing Northern Ireland to stay in the EU while the rest of the UK leaves.
Perversely, the Backstop is designed to be a legal guarantee that a hard border will not return - but, unfortunately, because the WA is legally binding and negotiations on the trade deal that would render it obsolete haven't even started yet, it puts the UK in an incredibly vulnerable position that, whether by design or accident, could lead to a permanent, de facto break-up of the UK. Ironically, the Backstop has become the main reason why a No Deal Brexit (and with it the possibility of a hard border) is so much more likely.
I guess another answer to your question is that most people in the UK never thought that the EU would be so intransigent when it came to the Irish border issue, and put the integrity of the Single Market ahead of all those other, arguably more important, considerations... but that's the way it has played out.