Similar argument can be applied to China. This is precisely why Zuckerberg is learning Mandarin and networking there.
How would the EU shut you out? Would the EU actually dare to begin censorship?
No, of course not by censorship. Through: fines for monopolist practices (happened to Microsoft), general smear campaign (happens to Amazon in Germany over working conditions), poking with a stick (the "right to be forgotten"), or just plain old taxes (the new "internet tax" is a current topic in the EU). There is always a way if you are determined enough. Politics.
It's not about censorship. It's about doing business.
What then happens if a competitor is established to take your former position in the European market - chances are they're not just going to stay in the EU. They're going to eat your lunch elsewhere too.