FPS, let me preface what I'm about to say by this: I badly wanted Bernie to trounce Clinton in the Cali primary, and following that blowout, win Clinton's superdelegates over to backing him based on his superior electability. But it didn't happen.
Now, in response to what you say here:
(1) Warren has been roundly attack-dogging Trump for about a month now. She has demonstrated her chops to take on the role, with vigor, for Team Hillary in the upcoming general election.
(2) Re Warren versus Feingold as progressive-wing leader: Feingold is, unfortunately for his fans, past his political prime. The press doesn't care about him, *and* he is waaaaaay too verbally meek in the heat of media-dominated political promotion. So, he really isn't even close at this juncture to being as useful for the Dem ticket this election cycle.
Perhaps Feingold will resurrect his national political relevance sometime in the future, but this time around, he would at best draw a few hardcore progressives out to the polls who would otherwise idealistically rail (at home) against Clinton's cynical recruitment of Warren to provide some cover for her otherwise wholesale embrace of unlimited campaign contributions.