There are several things the EU can do. First of all, they can freeze and/or seize Google's assets in Europe. Like bank accounts and real estate. Or even more important to Google, all of their European datacenters.

The EU can also disallow sales of Google's products, like any android phone. Or products like the chromecast, and don't forget Nest and Google Home.

And selling ads in Europe from a foreign country isn't too easy either. It is a lot harder to do business with a company which is on the other side of the ocean, and officially doesn't have a precense in the same economic area.

If Google leaves all of that behind in Europe, and is not really allowed to do business in Europe anymore, soon enough there will be a competitor that stands up and can do things better. Especially if they can actually open offices in Europe and do business without major hurdles.