Generally, I think that historically that would be a reasonable expectation, but I think the current zeitgeist is a bit more ClownWorld than that. Losing the progressive wing will largely lose them permanently. In the sixties, the rally around Vietnam, and then the collapse of that cause de celebre as a rallying point sort of broke the back of the left movement. It was too focused, despite all the free love, Marxism, communalism, feminism etc., there just weren't as many True Believers. The utterly indoctrinated were far, far fewer. This time around, the over-reliance on OrangeManBad won't have the same effect. When he is gone, that vacuum will just be replaced by another rabid, desperate issue. There is a massive group of progressives who are now permanently and irrevocably immune to change and reason. They are political crack-babies, essentially, and their cognitive landscape isn't going to change. Just like Cenk on TYT, the second Biden loses (assuming he does), they will turn on Biden. They second someone else wins in 2024, they will turn on their own again. There are millions of people who have permanently lost all analytic ability and are coasting for the rest of their lives on a myopic endorphin rush of rage and self-righteousness. The DNC will likely discover that either this fall, or in the mid-terms, and realize they need to let go of the tiger's tail, no matter how painful. The DNC will of course paint the progressives as right wing crypto-fascists at that point, and try to lay their ideology at the feet of a nebulous alt-right movement, or if it seems fitting, call them all Russian/Chinese assets.
I mostly agree with this, except the fixing economic mobility via some new deal scenario. The way to make economic mobility more fluid is to a) get more manufacturing domestically -- I think via two-birds-with-one-stone of massive tariffs against foreign manufacturers who have a deleterious carbon balance, and simultaneously b) defund all post-secondary education outside of STEM and the trades.
It revitalizes and energizes a new middle class, rather than a wage class and a parasite class, and the market will reward people, ideas, innovations, and businesses that offer something better, just as the limited Internet economy allowed a bunch of fucktards to get rich quick in the nineties.