Disruption doesn't only happen from a head-on challenge. Effective disruption is often not considered relevant until its way too late, and often couldn't be acquired even if seen. Consider how Craigslist destroyed the newspaper classifieds business, and that buying Craigslist would not have helped a newspaper at all.
Of course this will likely happen when a decent alternative presents itself. This is not guarenteed to happen though. The same could have been said about 'search' when Google wasn't around. 'Yahoo will buy them'. They had the chance but did not see the potential Google had as a searchengine. Whith this in mind it could potentially happen that a new company comes up with a better idea and grows too big too fast for GOOG to buy them. But that's the whole idea behind investing in the stock market: past revenues are no guarantee for the future...
None of these once up-and-comer's markets were properly established before their respective companies came along. AdWords is an established product in an established market-- Google's on the watch.
And then how Yahoo failed to compete with AdWords because of corporate politics, worries about existing revenue streams, etc.? http://www.wired.com/wired/archive/15.02/yahoo.html
Disruption doesn't only happen from a head-on challenge. Effective disruption is often not considered relevant until its way too late, and often couldn't be acquired even if seen. Consider how Craigslist destroyed the newspaper classifieds business, and that buying Craigslist would not have helped a newspaper at all.