TL;DR version: In order to get what you want, you have to be ready to fight for it to the bitter end, so Bernie and all his supporters should just give up and stop trying to change the system.
So, I supported Bernie in my state's primary, and I fully acknowledge that barring something catastrophic he is probably going to lose. However, I also support him continuing the campaign not because I think he will win, but because it best serves the cause he is championing. Because, despite what the linked article claims, Bernie's not going to disappear into the ether as soon as the primary's over.
Think about it. Before this campaign, few people knew who Bernie Sanders was. Those who did really only saw him brought on to cable news talk shows as a bit of a sideshow act so the audience could gawk at the spectacular old Jewish socialist senator. Now, even those he's almost certainly going to ultimately lose the primary, he's gathered enough support that people are being forced to start taking him more seriously. He's already stated in so many words that, regardless of the primary's outcome, he's going to stay in the Democratic party, so if the Democrats take back the Senate in November he's going to be well-positioned to compete for a big leadership position. If he ends up in a leadership position, he's going to continue getting airtime. If he continues getting airtime, it creates an opening for more progressive candidates to compete for seats on the federal, state, and local levels.
And that's how you start a revolution.
The thing is, in order to do that, he has to prove that he and his supporters will no longer be ignored. In order to do that, he has to continue to demonstrate that his political views have a broad base of support. In order to do that, he has to stay in the race. Dropping out now just gives the conservative Democratic establishment carte blanche to completely write off the progressive base and continue sprinting to the right like they've done for the past few decades.
So I support part of what Drum is writing about. In order to change things, you need to be ready to fight for it. However, contrary to what he argues, doing that means continuing to challenge the Democratic establishment in order to pull the party kicking and screaming back to the left. It's hilarious that he brings up the New Deal as an example of change you have to fight for, considering that the modern Democratic establishment has worked hand-in-hand with the Republicans in order to dismantle it and Bernie's one of the few people in government who's trying to bring it back.
In essence, the left wing of America needs to learn a lesson from the Tea Party and force the Democratic party to start taking them seriously.