I think this would still leave Google with a fairly strong argument - if Bing does it for all search engines, then they're effectively copying whoever is most popular. Since it's done through Internet Explorer, which is still bundled with Windows in most places, they could try to make the argument that Microsoft is using their position in the OS market to crush competition in other markets.
Interesting angle to go through the tied market and competition policy: that's a type of authority that is far more intelligent, and precisely just prosecuted IE in Windows. However, you'd have to either have a US court acknowledge that a European was right to disagree with them in the first place, or have a European court admit that their previous decision wasn’t enough. It’s feasible, but hard.
Where you’ll be more limited with it, is that it’s apparently not IE, but the Bing Bar that is at stake—the connection is getting thinner.
