You're missing my point entirely...

Forget about BiS builds right now. Even with a BiS build that has absolutely no Parry on it, you natively have quite a bit of Parry.

That amount is gonna be converted into Tenacity, which will in turn affect DPS. By how much, we cannot know, only SB's release will tell.

You're effectively gaining a DPS source where there was none before, and it has absolutely nothing to do with the gear you are wearing, cause even without any gear, you'll have a base amount of Parry (0 DPS) that will transform into Tenacity (additional DPS).

I'm not saying the change was made ONLY for this reason, but it's folly to think these changes aren't balanced around each other. My theory is that tank damage output is gonna be more of less the same, or at the very least it won't decrease nearly as much as people think it will.