People said this about weed. We literally had two states add it in like the last 10 years. Once a few more states pass RCV via initiative we'll start seeing legislatures take it up on their own.
Well the point is that lack of 2 party support doesn't mean it won't happen, it just means its a slower, longer push.
Edit: I would also say there's likely less built-in opposition for RCV - even hard conservative states like idaho are fighting ballot initiatives to expand RCV this year. 2 states are voting on it. Only 9 states have banned RCV (vs federal bans for MJ)