Well so the only difference is the CR of monsters? It seem to me a poor way of define high level adventures. Maybe I'm an old school grognard, but the adventure to me is to face great perils with little means. Hi level are there to be a target, the classic bait to move on the PC, but more important is the journey. Arriving at high levels my interest for the adventures is less and less and many narrative forms that could be used in this tiers are simply not working in a RPG. it is not an accident if all the narrative must have a strong motivation, a great danger, a formidable opponent. Player's must believe that their abilities are not enough, they must fear the fail of their task and the end of their life. is not an accident if Tolkien mixed powerful beings with very weak one and put in the hands of the weakest the key to save the day. LOTR works because trying to enhance the power would be to help the enemy, power is something to escape from. Well DnD mechanics are based exactly on the opposite and this is functional and funny for a RPG to move on but unfortunately leave us with a very few things in hand when you are close to reach the top of power.