- We weren't fine before joining the EU; our economy was in a bad way and we were desperate to join the EEC (as it was in 1973) as a result.
- The world has changed massively since 1973; globalisation means it's rare for smaller countries (such as the UK) to make products on their own any more.
- The EU is our largest market and we can't replace it with other countries.
- Saying "We were fine before the EU" is like saying "We were fine before electricity or cars"; it ignores the progress made possible by the UK's EU membership.
It's often stated that "the UK was fine before we joined the EU so we'll be fine after we leave!" This ignores the fact that a) we weren't fine before we joined and b) the world has changed immensely since we joined, not least with the true advent of globalisation. We don't make things on our own any more, rather we make them in collaboration with other countries and there naturally has to be an international legal framework to govern this. The UK (as a country of some 65 million people) lacks influence on the world stage compared to larger and more powerful countries such as the US, Japan and China and we overcome this by cooperating with our near neighbours to all our benefits. If we didn't then we'd have to accept EU and other international laws (assuming we want to participate in global trade in any meaningful way) but with hardly any say in making them. The US has practically confirmed this by saying we would have to accept their food standards as part of any post-Brexit trade deal.
The UK has benefitted enormously from our membership of the EU but before joining was "the sick man of Europe" with an economy that was growing more slowly than the members of what was then the European Economic Community (EEC). Indeed this why we were so desperate to join the EEC - it is emphatically NOT a case of "being hoodwinked" and "we were only joining a trading group"; we were looking to get out of our rival trading bloc, the European Free Trade Area (EFTA) because it wasn't performing as well as the EEC and we were "losing the race".
In the early 1970s the UK earned the somewhat unwelcome nickname "the sick man of Europe" because our economic growth was being held back both by the decline of traditional markets (the former Empire countries in particular who suddenly found it easier to source their requirements from neighbouring countries rather than the former motherland) and domestic industrial relations problems. The decline of the Empire/Commonwealth as an export market arguably had a hand in causing economic and social hardships such as the three day week, miners' strikes and occasional power cuts and provided evidence to the government of the day that the UK's future lay in collaborating with our European neighbours as our former colonial territories increasingly turned their backs on us.
Regardless of the historical context, the world is now nearly 50 years more developed and more integrated than the early 1970s when we finally got to join the European Communities. Globalisation is a fact and is only heightened by the massive advances in communications technology since the early 1970s. Historical success is not a guarantee of future performance in a global context! If this were true then the Roman Empire would ever have fallen.
The UK isn't ahead of everyone in production and science anymore. The UK doesn't have massive overseas assets holdings anymore (because when we did they were gained by suppressing other countries and their people). The reason the UK was the fifth largest economy in the world for so long was thanks to our extensive trade links and financial services sector... Trade which the UK got through the largest free trade union of all time (the EEC/EU), and a financial sector that's fleeing the country. Practically all other small to medium size countries find it necessary to join regional trade and power-sharing alliances in their geographic corners of the world. There is no reason to suppose the UK can buck this trend. The EU countries and their single market remain our most important trading partners and this cannot change no matter how we might want it to.
To say we were fine before the EU and will be fine after it is to ignore all the evidence of why we were so desperate to join the then-European Communities in the first place and also the mass of evidence from the behaviour of other world countries, the global economy and global trends since we joined the EC in 1975. It’s like saying “We were fine before electricity or cars so we’ll be fine without them!” It ignores the real life-changing effects that electricity and personal transport have had on our lives even if we take them for granted right now.
If as a country we retreat into isolation then it won't be splendid isolation - it will just be isolation. Or we can maintain active links with the EU like Norway and Switzerland do, but at the price of accepting EU laws without having any say in making them. The best deal is what we've got now - being an active and influential member of a continent-wide alliance with a real say in making its rules and shaping its policies.