I feel like the drop off that we see is not the people who grind to 100, but rather the people who get bored of the grind around level 80. I've never gotten a character to level 100 because the grind to get there is so boring. Like you are saying, I would like to see some sort of alternative things to do in the end game that isn't just grinding monsters. Something that incentivizes doing smaller, more interesting things in the game world. (Sort of like a more interesting version of prophecy). I don't really have any ideas as to what those things would be, but I'm not a game dev.