I recently talked to a player who managed to solo most, though not all, of the Glory of the Hero achievements from Wrath. He mentioned ones like Share the Love that been impossible to complete without putting a group together, and as you can imagine, getting four more players for heroic Wrath achievements isn't the easiest thing in the world at 85. People who want the achievement for their alts are rarely interested in doing a meta piecemeal, and this is probably going to get worse, rather than better, once account-wide achievements go active. Once someone's got a red proto-drake on one character, they almost certainly won't repeat the process on one of their alts.
The vast majority of achievements get easier as you level up, but the canny achievement hunter will pay attention to the ones that don't. As the game changes, players' priorities and interests change, and older content has a tendency to fall by the wayside. It's best to strike while the iron is hot.