"Why have so many fallen out with the EU gravy train"
It a global anti establishment trend, it happened with Trump in the US, Macron in France, the Arab Spring, getting rid of the old guard.
Are the days of the EU numbered?, not yet, too many vested interests involved, at its most basic Northern Europe being contributors, South and East recipients. Germany and France and the others subsidise the south and east so they can buy cars and other industrial products. The subsidies come with strings which may not be welcome, it's their choice.
What matters is the EU unity now, not at some time in the future, at present it is very strong and it is totally futile for leavers to use this argument. Our destiny will be set in the next few months and will be permanent, either in or out a situation involving continuous haggling year by year would be madness and I'm pretty sure the EU would not agree to that sort of deal.