I suppose leftist and socialist are a bit different here, since I think leftist gets used online as a pan-left term for social democrats, socialists, anarchists, etc. which are distinctions that probably won't ever exist in the wider public. But I don't think it's impossible that we'll see socialist become a much more mainstream political term as millennials become a more powerful voting bloc.
The joke about 'alt-left' before Trump used it that I'd hear was that centrists needed something to red-bait with since socialist wasn't doing the job anymore. I think that'll become more true, especially if more Democrats being running with the socialist label.
For what it's worth I replied to the author with the same skepticism and his response was basically that he thinks threatening abstentionism is a much riskier strategy than primarying leadership, specifically citing what he considers the success of Zephyr Teachout's 2014 challenge of Cuomo as a failed primary that still succeeded in moving its target office to the left of where it would be without the primary, and that instilling the fear of losing a primary is the best way to keep politicians in line.
I'm not sure how much I agree with it, but that's his response.
