I think it's less about "Apple would own the mobile space", but "some other company than us would own the mobile space and would be a threat to our advertising

I think it's less about "Apple would own the mobile space", but "some other company than us would own the mobile space and would be a threat to our advertising business". Just like Microsoft owned the browser market in the late 90's and 2000's, and it was a problem for Google. That's why they poured resources first into Firefox, then into Google Chrome.