All states have a right to leave the EU by treaty - it was inserted to defuse a previous bout of Euroscepticism that argued by analogy with the US civil war that union author a secession right was tyranny.

I can't see its use being refused: that would disintegrate the bloc. And there was no question that we (the Brits) wouldn't be allowed to, so the precedent has been set.

I would imagine a few tweaks to the Article 50 exit mechanism will be made next time the treaties are revised - there are some game theory problems in the current trigger mechanism and the timescales set down turned out to be too short in practice - but it's no longer in dispute on principle.