Have you ever heard of innovators and early adopters? Do you recognize the influence they have in markets? There is a recognized "product diffucion curve" where innovators and early adopters have significant influence over the larger (mass) market.
I would assert that early adopters are more likely to use add-ons. I would therefore assert that the lack of add-ons may be important, and may be statistically significant. I would also assert that Firefox, IE and Safari meet the criteria of "good enough" and therefore replacement products have a much higher hurdle to achieve market penetration.
I would also assert the market share numbers show a problem for Google. A spike in usage followed by a decline as they are seeing indicates a problem in "crossing the chasm" (search for the book of that name if you don't follow).
However, all of this is simply "assertions" and not "proof" - same as your assertion that add-ons are not an issue. My primary point is that meeting the needs of the innovator vs. the mass market is often underestimated (and is a very tough balancing act)