Where has the US hosted an Olympics other than Atlanta or LA? St. Louis? Come on. Bidding culture is a little different now in 1904 - and back then, St. Louis was Dubai, the next great world city.
The IOC are a pretentious bunch of royal twats. They love cosmopolitan metropoles or scenic locales; they look for the kind of host that would make the cover spread in Travel + Leisure. They've dropped endless hints they would love to see an SF Olympiad. It's an open secret they hated Atlanta. I can't ever see them going for Dallas, no matter what minor points can be marshaled in its favor - and "it'll inspire transit in the US south" isn't the greatest one. Also, don't forget most of these people are European, and they still associate Texas with Bush and everything poisonously unilateralist about the US - not exactly the best memory to pair with the Olympic ideal of international comingling.
On the other hand, I can see Philly maybe having the history, scenery, and story - the need and clear potential for a "Barcelona effect" - to sway them. The potential of a Philly bid might even be what would knock out Boston; you're right that hosting here might be too safe, leave too little of a legacy (though, horrifically cynical as it may be, portraying this as a "comeback" from the bombings of the marathon - a sporting event - might work in Boston's favor). In any case, that's to say nothing of the NIMBYs eventually sinking the bid, somehow, before things even get nearly that far.
