I cannot wait until WoW is available so I do not need to use the battle.net client.

This is unlikely to be the case; Valve supports third-party launchers via Steam, so most likely this will not change the dependency on that

Overwatch on Steam does not require b.net, so I do not see why it wouldn’t be the case with their other titles.

I do agree with having to repurchase. I don’t really play D4, so I might just wait for a big sale if I do go with a repurchase.