Notice I didn't claim, or really even imply, that socialism has succeeded (historically or otherwise).
Do ideas, good or bad, only get to be formed and expressed by those with jobs? What if Bernie is elected and injects some socialist principles into our capitalist economy, and people are willing to give it a try, and it works? The middle class grows and strengthens so people who describe themselves as middle class no longer are a paycheck or two, or a serious health issue from effectively being broke and permanently indebted, the health care system moves toward a single-payer system (that works well elsewhere), public education miraculously gets back to where it was 30 or 40 years ago. Does that make Marx's ideas valid given Sanders will be employed?
I'm not saying Sanders is the answer. I'm just saying that capitalism may need an injection of some socialist principles to function well for everyone (or as close to everyone as possible).