Well I can't argue with that, but I believe that despite a mega evolution regular gyarados still performs the role of a late game cleaner fantastically and is criminally underrated, which can also be said for mega gyarados.Bulky Leftovers Gyarados is really cool, but as a sweeper/cleaner/whatever you want to call it, Mega Gyarados (or, rather, having the option to Mega Evolve) is definitely superior.
The typing mind games are amazing -- they often lead to it getting multiple Dragon Dances or getting one in a scenario where it otherwise couldn't (e.g. against a Stone Edge user, like Tyranitar). The worse typing doesn't really matter defensively, because it's generally only going to Mega Evolve when it wants to remove a weakness (so it can set up) or when it starts to attack. Speaking of setting up, the increase in bulk is pretty ridiculous (+30 to both base defenses, not to mention it still gets to use Intimidate). Mega Gyarados is also a lot stronger then Gyarados, unless Gyarados has a Life Orb, which cuts into its bulk even more. OHKOing Rotom-W (after SR or a bit of prior damage, which isn't too unreasonable to expect) is huge, and Mold Breaker lets it pull off other shenanigans like beating Quagsire and OHKOing Dragonite through Multiscale. Scarf Rotom-W is also pretty rare.
Why is that anyway? I can't seem to get my head around why mega gyarados isn't more common.