… What? This is a ridiculous counter-argument.
For one, shaman’s could tank at the very start. They just weren’t given a formal role to later.
Two, shamans use mail – a better armor type than leather. Of which we have several tanks using leather already (monk, druid, demon hunter).
Three, shamans use shields.
Four, shamans can literally call upon the powers of EARTH to defend themselves.
If I can make a skimpy kawaii elf chick to guzzle some beers and face tank a dragon, I see little reason why my bear that can call upon the earthen spirits themselves shouldn’t be able.