I always thought that talk of ever closer union was scaremongering just like the threat of an invasion of people from Turkey. Possible in theory but the reality is somewhat different.
Trade outside of the EU isn't impossible, Germany sells three times as much to China as we do. If we were really worried about that we would have done something about it years ago. The reality is even in the modern globalised world most of your external trade is going to be with near neighbours.
Personally I think closer union is a good thing in principle but the fly in the ointment is our lack of fiscal discipline. If we had been in the Euro we would probably have suffered as much as Greece has. Even if the vote had gone for remain there was almost zero possibility of us joining the Euro.
This isn't to say the EU is perfect, there is plenty of room for improvement, I just think on balance we were better in than out and there are severe risks if the whole thing collapses.
Firstly, just looking at the negotiated opt-out of the ever closer union principle, how legally binding those negotiations were was never resolved. There appeared to be some doubt about whether they would get past the European Parliament, and the Brexit referendum happened before any definitive position could be established.
Secondly, even if they were binding in theory, how would that have worked in practice? For example, 26 out of 28 EU member states are either in the Eurozone already or in principle required to join in the future. Is it really credible that so many member states would always remain honest and neutral on any policy that might disadvantage the Eurozone just to be fair to those with opt-outs (the UK and Denmark)? Similar arguments apply in other respects, the Schengen Area being maybe the next most obvious case.
Thirdly, that issue was distinct from membership of other EU-based mechanisms such as the Single Market and the Customs Union. The latter in particular seemed to be a big concern for the people I mentioned, not just because it precludes negotiating future trade agreements independently as widely discussed in Brexit-related forums, but also simply because while it makes trade within the EU nice and easy, it also makes trade with partners outside the EU a lot harder, because all members are required to impose the EU's standard barriers and tariffs on external trade.
Obviously whether or not you think ever closer union within the EU is a good thing is a personal/political view and reasonable people can reach different conclusions, but if you had thought it through and concluded that it wasn't, I don't think Cameron's negotiations would have done much to affect your logic or your final decision on that point.