And now the immediate implications of leaving the EU are much greater than the immediate implications in 1975 of joining a customs union, so even if a 50%+ referendum was enough for a customs union, a strong argument can be made that leaving the EU should be held to a higher standard.
I think the main takeaway of this experience should be this: the EU should never accept new members unless there's durable majority support in the member country for being a part of the EU. One plausible version of this is 60%+ support in two consecutive elections.
I think the EU should simply improve its outreach programs. Some of them were very successful (Erasmus), some weren't (the EU flag on buildings is almost imperialistic).
The truth is that getting 511m+ people to agree on things, when they won't even speak the same language, will always be a huge challenge. Unfortunately, there is no alternative: this planet has to learn to resolve its problems by talking and agreeing a common course of action, so might as well start with your immediate neighbours.
Now, other member states did hold referendums on these changes, of course - and in some cases, their voting public ended up rejecting those changes. They went ahead anyway, usually with some tweaks to the language used to describe them but very few substantiative differences.
That's utterly false.
Ireland (twice) got legal guarantees that the scary scenarios evoked by the rejectors, would never happen.
Denmark obtained opt-outs, again listening to concerns and guaranteeing that they would never happen.
France (and basically the Dutch) literally re-negotiated the whole treaty.
Those are substantive changes. I don't know how more substantive you can get than "if we make a law you don't like, you're free to ignore it" and "ok, we'll do it again from scratch".
Something I see forgotten often is the referendum experience in other countries like Canada and Australia (who have them more frequently) is that most referendums fail, it's incredibly hard to get a vote to change the status quo because you have to convince people why changing is necessary. A vote to remain in the EEC/EU should not be seen as the same as a vote to join would have been.
I suspect this is why there has not been a second brexit vote, they know how much momentum there is behind leave side. Well that and the decade of uncertainty of stay vote would bring.
There was no supermajority required to take the major and quite possibly irreversible step of joining the EU. Indeed, there was no referendum on it at all; the 1975 referendum was on leaving after we'd already joined, and came with the same terrifying warnings about the perils of leaving as the more recent one. The government of the time just decided to join and put it through parliament like any other law. It squeaked through the House of Commons on an incredibly tight 309-301 vote...