I'll speculate.

Some of this depends on world events between now and the primaries. Polls have shown that national security is the largest issue on the minds of those polled. Clinton and Trump have made this their turf. Clinton with some experienced rambling, Trump with conjuring boogeymen and talking about how he'd strong-arm them. And, it's not without some reason (Paris, San Ber'dino). If this doesn't flare up between now and then, the focus on this issue will wither enough, leaving the ever-present domestic issues.

Sanders needs to ignore the national poll numbers, and hammer the superior head-to-head numbers as the proof that he is the most electable Democrat. He needs to give more specifics when asked for them. He has the specifics. He's very alert in interviews, but during these debates, it's very "intro to Bernie" answers. I can understand why, but it's time to get more specific. Perhaps in future debates.

Then, with voter turnout that's more than just talk from younger voters (I believe the genuine enthusiasm is there) Sanders cruises. Momentum from the early victories will count. Clinton runs a dirty campaign, but how much self-harm is done between now and the votes is uncertain. If Clinton falls behind in primaries, it should get more and more dirty. People will turn away from it.

Let's optimistically (but not unrealistically) assume that Sanders gets the nomination. At that point, we have to give him a refill ($$$), and then, by sheer numbers and running a clean campaign, he'll cruise. Trump's rudeness will only grow less and less palatable (as rudeness does), the media won't tolerate him, he'll portray the media as biased (and not be wrong) - it will be very contentious. But, the Republicans don't have the votes to win a general election, no matter what.