I bet they've more than thought about it. I bet they have data that shows them their customers care more about using Google Docs than they care about using IE9, and will switch browsers.
I also don't think a company as big as Google would make such an important decision without considering the consequences. This decision wasn't made on a whim, they've clearly given it some thought.
Yep, I'm betting on unintended consequence rather than deep plan here.
It would certainly be pretty unwise to exclude the best-paying customers intentionally.