It's 5% for Europe on average, but for Germany and the Netherlands it's much higher (15% for the Netherlands). No politician in his right mind is going to risk that to spite the British.
Leaders of EU countries are generally not in their right mind though. Merkel has flat out said that the EU for them is an emotional issue not a logical one.
This vote will set in motion a chain of events that can reshape politics across the continent. The EU leaders who have little to lose will use their votes to force the UK's main trading partners (Germany, Netherlands, Sweden etc) to start a trade war with the UK, at the same time as the UK is wanting to sign trade deals. This will piss off the populations in those countries. Referendums may well follow there too.
