It's a long way to "capturing a significant portion of the market". What they can do is reach some of the Internet users in dire need of an upgrade, like IE 6

It's a long way to "capturing a significant portion of the market". What they can do is reach some of the Internet users in dire need of an upgrade, like IE 6 users who trust the Google name, and get them to use a reasonable browser. Not that that explains why Google had to invent their own browser. But their target market was average Internet users, so they might as well try to get them to run it.