The assumption that all the non-Warren voters are automatically going to support whoever's against Bernie in a 1v1 contest is or some other voting method is flawed.
The problem the moderates have is their candidates are all mediocre at best. Like, is any moderate in this thread even proactively supporting any of those people, rather than just trying to find the one's going to beat Bernie? Is there any moderate candidate who's gonna get people really passionate about them, and want to get out there and fight for them? That's why they're losing.
That said, yeah, they should totally do RCV. And hold all the primaries on the same day, Christ. It doesn't help anyone to have the party fighting amongst itself for months on end.