If Steam puts up a bunch of new Agreements, and you refuse to accept, you’ll continue using the non-updated version - correct?
In theory you could keep using the service that adheres to the old agreement - but they will only provide the service under the new agreement. So effectively, no.
Unfortunately, Diablo 3 is an online game - even singleplayer. In case of starcraft, it’s even worse - the only reason for it to be online is multiplayer (fair enough) and drm (boo!).