Agreed. One of the problems in pretty much every edition after the first is that the first MM is expected to include all the iconic monsters, and yet monster design tends to improve over time. This means that, a few years in, the iconic monsters are almost inevitably the least interesting (mechanically) in the game.
When 4e did the "Monster Vault", cleaning up and reissuing all those existing monsters, it made for a very significant improvement, and 5e would benefit from much the same thing, IMO.