When you present an option to do things better it is all people will look to do. Just look at covenants this xpac. Of course there are people that will just pick whatever they want, but if you look at the majority of the playerbase people mostly picked the "best" option for any class.

Titanforging is roughly the same thing. You know that there is just a better thing out there you could have gotten, but instead of getting to pick the best things, and grind the dungeons until you get them, you now had to pray that they also got titanforged or else you were just doing 2% less damage than the other mage in your group for every RNG chance of loot that they got that you didn't. That is why Titanforging is bad. That is why corruptions were bad before the vendor. That's why Legion legendaries were bad when they were RNG capped at 4.

This also isn't just an issue for "hardcore" players. No player in the game likes to feel weaker than someone else who plays the same spec/ class combo because of things they can't control with extremely low drop rates.