No-one suggested a sudden drop-off of exports to EU. A gradual decline, perhaps, as I've said, at an alarming rate.
Suppose any increase in 'replacement' markets is at a much slower rate. After all, if it was that easy to break into other existing established markets, we'd already be doing so!
So consider a scenario of our exports taking a dive, imports continuing, perhaps now more expensive, 'cos we're no longer in EU.
Balance of payments become more of an imbalance. Austerity drive by government, falls in tax revenues (hence why the EU payments won't be redistributed. They'll be retained to make-up loss in tax revenue)
Government devalues Pound to try to make our exports more competitive, fall in UK living standards, migrants decide they're better off elsewhere, (I can hear the Brexit campaigners cheering!). Public sector services start to suffer breakdowns.
Further austerity cuts, NHS, Education, Military, LA, Police budgets cut. Higher educated/skilled indigenous Brit's start to emigrate.
Tories given severe kicking at next General Election. (not 2020, too early, more like 2025 or earlier). Corbyn elected as PM.
Campaigns for re-entry into EU, and starts negotiation.
EU insists on further Austerity cuts, more devaluation prior to adoption of Euro, and insist on acceptance of Schengen Agreement as prior conditions to re-entry into EU.
If UK re-enters EU now at a much reduced level of influence, more like a developing country. It would take 50 to 100 years to regain the level of living standards experienced prior to Brexit.
What a pyrrhic victory for the Brexit bunch, caused by pious deception.
I can imagine the re-entry campaign poster: a picture of Stan Laurel, twiddling his tie (with the inevitable Union Flag print) and the caption: "Another fine mess you've got us into!"
Perhaps the Brexit would become as toxic as the Iraq war to the various political parties..