Introduce a new tank and make dark knight DPS again

[Saeno]

Can I be considering to be a negative player if I tell SE doesn't seem to learn the lesson?

I would say so because SE has done well to make tanks this expansion way more balanced than before. SB DRK isn't even remotely as bad as HW PLD.

I mean, there is only three tank in the game, I don't understand why they seem so similar and why everytime one is fixed, another is changed in a bad way...

Because they have one goal in mind with each patch in terms of role balance and sometimes they go overboard. The changes to DRK and WAR in 4.0 launch were extreme but WAR was fixed overtime and PLD was nerfed as needed. DRK hasn't received the attention it needed yet.