The EU needs us more then we need them and they will not be dictating anything on an economy as large as ours.
Both the Remain and Leave camps have over-inflated claims, in my opinion.
The UK is a big enough country, and a big enough EU net importer, that the EU can't just dictate terms. But the UK is nowhere near large enough that we can expect to dictate terms to them, and have an EU we've just left fall over itself dancing to our tune.
The truth is in the middle. The UK is big enough that the EU can't just steamroller it, without risking serious economic damage themselves, but the UK is very far from large enough to bully the EU.
Wolfgang Schuable (German Finance Minister) summed it up pretty well today when he said, more or less, an independent UK can't expect to leave the single market and still retain all the benefits without any of the costs, but it would be in both parties interests to reach a mutually beneficial trade deal.
Of course, neither Brexit nor Bremain camps can specify the exact detail of what that deal would entail, because it will be the result of negotiations that won't happen until after an exit decision. It's pretty safe to assume full market access as if we were still in is one extreme limit, and the same basic terms as far smaller non-European countries is the other limit, and the UK's eventual deal will be somewhere in-between.
It's safe, in my opinion, to assume exiting will incur some costs in EU trade. However, the exit argument is that there are many benefits that don't relate to EU trade, from trade with the rest of the world which will be freed up from having to rely on EU trade deals, to non-monetary benefits like "sovereignty" issues.
As for other countries leaving, who knows. The obvious candidate is Greece but they don't, or at least didn't, want to. But any future Euro crisis could force their hand, as could a worsening migrant crisis. Anti-EU sentiment is rising in quite a few countries, and if there's a really bad migrant summer, who knows if Marine Le Penne might even gef elected in France, and Merkel lose power in Germany. And a big external economic shock, like something unexpectedly bad in China, could trigger slumps or recessions, and either a Euro crisis or a Eurozone banking meltdown, many Euro banks having not even recapitalised to the extent UK banks largely have.
Ten years time could see the UK and several others out, or even a wholesale disintegration. Or no change. My bet would be either no change, or just the UK out.