What happens to progressive causes when centrist Democrats win? Well, sometimes we will win and sometimes we will lose. But at least we will win some of the time, as opposed to being governed by Koch Republicans, where not only will we never make progress, but we could see truly horrible policies enacted.
When I was young, a mantra among progressives was that America had to stop operating as global policeman. Vietnam was the signal episode of arrogant and ultimately self-defeating American overreach. But there were plenty of other cases of the U.S. government doing the bidding of oil companies and banana barons, and blithely overthrowing left-democratic governments as well as outright communists (or driving nationalist reformers into the arms of communists.)