As has been said several times above, we don't live in a vacuum, surely some exposure came early for Adams and I do agree that Adams got an education in photographic history but it also seems apparent that his inspiration came before his formal education or serious study of photography.
Galen Rowell and Joe Buissink I think are very reasonable examples of commercially successful photographers who like Adams turned a hobby into a successful vocation.
I believe that Rowell, Buissink, and Adams each have a couple very important things besides photography in common though. They are/were commercially astute to begin with and they each had/have passions/inspirations they wanted to share with the world.
These guys didn't start out to be visual artists. Adams was studying to be a pianist, Rowell was in the automotive business, Buissink was studying for a Phd in psychology.
Their artistic inspiration was driven by wanting to express/share their moments/experiences/emotions with others. Yosemite for Adams, climbing for Rowell, and emotions for Buissink. Photography in a sense for these guys was simply a convenient tool.
The important questions after the inspiration are present tense, like "what tools and skills do I need?" and "who is my competition?" not past tense, like "what would Stieglitz have done?".
I don't think we're talking about commercially successful photographers here - we can give plenty of examples of commercially successful photographers who neither advanced the medium nor understood a whit of art (or even photo) history. And I don't think that even academically trained artists, let alone innovators and game-changers, ask, "what would Stieglitz have done" but rather - "Stieglitz did this. I'm not going to do this, but something different...".