I don't think anyone can say what will 'very likely' happen, but NASA often hires many contractors and also does much work internally (e.g., at JPL) for a major project. Also, there may be other national space programs partnering in the mission.
Not saying the dream won’t eventually come true, but it if it does, it will take many decades. Come 2100, I think there will be a lot less than 1 million people on Mars.