I don't think it's a millennial thing -- every generation has a variation on cocky, entitled, rich, white, male douchebags who cruise through life on easy mode and think it's because they're brilliant. I can just as easily see these hucksters on Wall Street in the 80s.
I agree Marcus' convoluted plan (at least what we saw of it) made no sense and I had no idea WTF was going on most of the episode. But I do think his overall point was correct: the only thing they're actually good at is convincing people to buy crap on Instagram (and apparently, up until now, convincing suckers to invest in their unprofitable company), so they need to make that their business. They're not good entrepreneurs, and the socks will inevitably go out of style as they lose touch with the zeitgeist as everyone does. But if they can't use their social media selling abilities to shill products they don't personally love, they'll never turn it into a marketable skill.