The Dothraki would crash against the walls of King's Landing and break down, one by one. The mighty power of the Lannisters, Baratheons, Starks and Tyrells combined would surely overwhelm the horde, no matter how skilled they are, and that would leave Daenerys in grave danger. Without the Dothraki, Daenerys is just a young girl without any political experience or nous back in Season 1. She'd have been captured and almost certainly killed once discovered purely to end the line of Targaryens once and for all. With Daenerys gone, Westeros would gradually return to its former state, although the race for the crown would still sizzle.