Society makes heroes out of entrepreneurs and adventurers that tilt at windmills and succeed. Napster slays the music industry! Twitter comes out of nowhere!
The thing about taking on the biggest giants is that most of the time (so often as to be all of the time if you're willing to do some rounding) you fail. You don't just fail at the end, you often fail long before the end.
Yet the dreamers persist. These are usually the garage entrepreneurs, people with little market success behind them, those working without a track record or significant resources. People forget that Google was backed with millions of dollars from the biggest VCs in the world when they took on Yahoo.
I know, I know, I'm supposed to be the guy who says, "go for it!" but the fact is, most of the time the choice to take on impossible odds, to challenge the entrenched monopolist is the work of the lizard brain. After all, if you dream the impossible dream and go after the thing that can't possibly work, you don't have to worry about being criticized, you don't have to worry about the responsibility of shipping or serving your customers. After all, it was impossible.
Tangling with the largest possible opponent, when you are severely overmatched is a way of giving in to the resistance, of not actually shipping.
My best advice: win little battles. Get in the habit of winning, of shipping, of having customers that can't live without you. Once you've demonstrated you know how to do the art, then go after the windmills.