Even if they made tanks to be exactly like a DPS as you suggested, and even tuned them to perfection (to not be broken nor useless), you will still have the same 1700 Andy come here after losing to one and cry 'tanks need to be removed from PVP entirely' simply due to the spec icon and the perception associated with it, whereby its not the 'norm.'

Regardless, tanks suck now in Arena as Prot was the last viable one left. Blizzard kept buffing the hidden Tank damage taken modifier over the last 1-2 years due to Guardian and Prot. I couldn't even imagine how awful something like Brewmaster or Prot War would feel in PVP with 50% increased damage taken and virtually no spell mitigation.