Honestly, even if Sanders had managed to eke out a victory, it's not clear that his presidency would have been good for progressives in the long run. I'm sure he would have accomplished some important things, but his presidency, as a whole, would have been a damp squib. His policies would have been blocked by both Republicans and his own party, and these failures probably would have been spun as the failures of Socialism as a whole. The corporate media would have been relentless, and 'Brezhnev Sanders' could be primaried in 2024.
If he resorts to issuing executive orders in order to bypass the limitations imposed by his own parties, he would be painted as a tyrant along with the entire progressive movement.
The truth is that the Democratic and Republican parties are how they are because their voters want them to be. It isn't just corporations that have benefited from tax cuts, so have many wealthy suburbanites. Things will only change once the next generation of voters—which has been badly hurt by the current economic policies—dominates the electorate and votes according to its economic interests.