Even East Germany had "opposition" parties, and that's exactly the niche the GOP wants to be in.
And the Democrats NEED the GOP, at least for a while, to set the limits of acceptable dissent in "Our Democracy™".
If the regime can keep punching Mitt Romney and Susan Collins types as "fascists" who they regret that have to allow to speak, it makes it easy to push out or even round up actual opponents of the state - with help from Romney and Collins.
They won’t have enough voters to be a viable national party anymore. I think that means that they will be finished. You very correctly describe their function in our political system.