Chris Scott: When the UK joined the EEC in 1973, there was no single European open skies arrangement. That came much later.
In 1973 there was a network of bilateral aviation agreements which throttled competition, and going back to 1973 is probably unfeasible (the EU as a whole would negotiate any new post-Brexit agreement). Who wants to go back to the old world of bilateral monopoly arrangement and astronomically high air fares?
Brexit a problem? Just put the clock back 45 years. I love it!