Perhaps I could've been clearer. There will always be changes, hence, anyone who wishes to claim there's been a change can.

That doesn't obviate "once in a generation", does it? Perhaps they should've been clearer and said "once in a generation, and if the UK has a referendum on EU membership and we decide to leave, and if…"

It would've been more honest, at least.

I'll also note, you're right that the vast majority of leave proponents wanted a Norway-style deal, it was remainers who presented this form as part of their machinations to try and wangle the UK staying in. Don't blame anyone else for that.

Why does the statement "once in a generation" completely obviate any nuance? It makes sense to evaluate things based on events and changes in situation and context. It makes sense to not bring up things over and over again in an attempt to change something that people don't want changed. It doesn't make sense to use a phrase as the sole justification for avoiding reevaluating a decision made in a different context. I don't think it's rational to read "once in a generation" as "once in a generation, regardless of any and all circumstances, with no room for any reconsideration ever".

Any statement should be considered to be taken in the context that it is given, and constrained by rational, reasonable bounds and circumstances. Assuming anything outside of technical and legal language to mean the most absolute and literal interpretation of the words isn't productive or useful in most cases.

A politician says "once in a generation". In that time a lot of things can happen. Think back a generation (about 30 years) and of the big things that have happened. The SNP leaders, when making this statement, were all well over 30 and hence, knew that big things happen in that time.

That's why the changes you might say would lead to re-evaluation do not.