Its very good news if Steam accept it and Valve will implement it soon , very good news for all of us, but if we attract millions of users that means the network cannot handle it quite efficiently and that also means that bitcoin will go mainstream and its value will skyrocket. I don't think millions of users will start adopting it just because 1 or 2 big merchants starts accepting it.
I really don't have idea what will happen in bitcoin if all the people worldwide will start using and investing bitcoin. All that I am thinking now that it will skyrocket its price and will possibly to cost thousands of dollars.