Mortal Strike itself hasn't changed in years. The reason why Mortal Strike is being seen as such a problem nowadays, is because with how tough players have become while decked out in Legendary Artifacts, Mortal Strike is more important than ever to land a kill. Unless you're running a gank squad, you're not going to be able to burst down someone that has 150 Health, 45% DCI, 30% Eaters, 18 HPR, 50% EP (and carries 20+ Greater Heals/Cure Pots) and 12% CF. Combine GM+ Parry with that, and you're totally not bursting them down.
The damage output of Mages really hasn't increased much with Legendary Artifacts, but their toughness has. The damage output of Melee hasn't increased much with Legendary Artifacts, but their toughness has. The damage output of Archers/Throwers however, has significantly increased due to Legendary Artifacts, because they can now hit the vaunted 1.25 sec swing speed with 60% SSI/210+ Stam.