True. The Ubisoft games on Steam launch the UPlay app in the background, so it'll probably be the same.

OW2 can already run without Battle.net running, but launching it through Battle.net takes care of auth for you. Presumably they will modify the Steam OW2 client to include the Steam lib and handle auth via Steam. I think it's actually reasonable to have hope that Steam can launch into OW2 without B.net.

I'm sure they will just add steam account linking to battle.net and treat it like any other platform.

As does EA Origin.

EA with it then you can't play PvZ Garden Warfare.