Sometimes a quantitative difference becomes a qualitative difference. For example, motorcycles are just faster horses (in some ways worse, like for off-roading), but you could hardly argue they are the same thing. I would argue that google is substantially more useful than altavista, or dogpile, ever were.
And ultimately Altavista and Dogpile are irrelevant, because neither existed just a few years before Google. Web search is revolutionary; search engines before Google were just halting steps on the way. That is, the contention isn't that Google revolutionized web search, but that web search revolutionized the world.