Sigma has confirmed in a recent Blizzard post that the tank homogenisation seen on BFA alpha is in fact intended:
https://us.battle.net/forums/en/wow/...2287277#post-9
All tanks will boil down to having an AGI/STR ---> armor button and very limited magic mitigation tools. I think that this is the wrong way to go and makes tanking extremely one-dimensional. Tank balance will simply boil down to whose 1 button is better.
Legion arguably had the worst tank balance in the history of WoW, but BFA is shaping up to be far worse. Putting all the tuning eggs into a 1 button basket is going to amplify the smallest of imbalances. All classes are going to play exactly the same, and the power of utility is going to be amplified.
And with Blizzard's hands-off approach when it comes to tank balance, expect an entire expansion with 2x the tank with the best 1 mitigation button.
I am genuinely horrified at the direction tanking is heading in BFA.