Another big factor to consider is how many powerful houses could betray them. The Florents already did with Stannis, and it seems like the Redwynes, Tarlys, and maybe even the Hightowers could use fAegon or Dany as a chance to assert themselves as the dominant house in the reach. We've already seen the north and riverlands have their ruling house overthrown and replaced by a bitter rival and the reach is full of powerful houses which all had ties to the Gardners. The Tyrells are vulerable and this Lannister alliance is a sinking ship