In fact, it's probably Android that has put them over the edge. Gmail still has a fragile position. At least as far as my browsing habits are concerned, using Gmail didn't lead me back to google search or any of their other services.
Could you imagine what the world would look like if somehow in 4 or 5 years Firefox OS ended up eating Android's lunch?
Search is a major component of their advertising business, which these days is their core business. If people switched they'd definitely be in a major pinch.