I don't think anyone expects him to win the actual election by this kind of blowout (these numbers would probably be 350-400 in the electoral college).
It's the trendlines that are worrying - Trump has been trending up against her to the point of drawing even (+10->+3)while Bernie's downward trend has been less pronounced (+15->+10)*. In isolation her fall could be explained as the bump from Trump getting the nomination and GOP consolidation but the fact that Bernie's numbers haven't fallen correspondingly mean there's more to it.
Which brings us to his support (and her weakness) among independent voters. Infi.Boredom have argued circularly and endlessly about this but this is what I believe:
1. Many of her base voters (minorities) will vote Dem regardless of the nominee, the spectre of Trump should be enough to motivate the entire traditional Dem base (especially women)
2. On the other hand his base (youth) might be de-motivated without him, too anti-Hillary or too progressive (read: Jill Stein) which could cost her
3. Independent voters have, throughout every poll, and every open primary (even in Georgia!), shown a marked love for Sanders and dislike of Clinton.
Against this you have the fear-mongering capacity of the entire rightwing/Fox machine (tax!) which will absolutely affect him, but you also have public support for his policies (example - new poll:
http://www.politico.com/story/2016/05/poll-health-care-bernie-sanders-223206). Note that Obamacare, which Hillary is wedded to and is expected to rise in costs right around November, is less popular than simply scrapping it and both are less popular than single-payer.
Basically I am doubtful whether the fear-mongering can damage him enough among independents to change a double-digit lead to a
loss while a lead of 4 is super-vulnerable, based on the economy.
*RCP hasn't updated with yesterday's/today's polls, it has the numbers as +13 and +5.7.