Better mixed bulk is a moot point when the overwhelming percentage of priority attacks are physical and that Aggron has Filter and a mono-Steel typing to protect him from revenge kills. Not to mention Mega Aggron's physical defense and typing are leagues better than Tyranitar's, while attack is comparable enough that they still OHKO or 2HKO most similar threats. Mega Aggron is not outclassed whatsoever. It's insane physical defense, which is more important when in comes to revenging because of said priority moves, compounded by Filter and no 4x weaknesses means it's one bulky mofo. Tyranitar's MAIN pros over Mega Aggron are that Sand can handle Focus Sash users and that it's STABs are way better. Otherwise Aggron is MUCH harder to revenge kill.