Just a little technical speculation:
AFAIK, Google relies on a network of cheap decentralized PCs spread all over the world. If one breaks down, it's cheap to replace it.
Now we've seen the rise of Linux, P2P networks as well as wikipedia.com , where non-commercial users have bundled their resources and capacities in order to build decentralized networks.
If you've followed projects such as seti@home or similar projects, think about the following: why shouldn't private users decide to build a non-commercial, decentralized search engine in the not so distant future?
Also, one of googles main advertising partners so far is/was ebay. Couple of days ago, ebay bought shopping.com, IOW, their own search engine.
Actually, google's business model strongly reminds me of a
similar company that was founded a few years ago
Edit: I think the most important bubble to follow currently is the housing bubble in the US & UK