Trump's been leading the Republican polls for some time, though he still has a pretty small overall share and Carson has been surging recently. Its pretty hard to justify a claim that its clearly heading toward any particular winner on that side (particularly since its not decided in a national election, but state-by-state elections to assign delegates in a manner which isn't proportionate to share of vote, and there's not frequent enough state-by-state polling to have a clear picture of even the current state of those, much less where they are likely to be by the times they are held.)
On the other hand, while Sanders is doing well in some places, Clinton's still got a about a 20 percentage point lead in most national polling; she's hardly "imploded" the way Bush, widely perceived as an early Republican frontrunner, has. Many of the same concerns exist as discussed above with the Republican ticket as far as drawing conclusions go, but what you can read from polls doesn't show anything like a clear course to a Sanders win.
If you go back and look at the political news articles from 2007/early 2008, Obama was viewed as a long-shot. HRC tends to do worse the more exposure voters have to her, while Sanders is doing the opposite. It's going to be a tough fight, but I think Sanders has a great chance of winning the primary.
1) It's way too early for polls to be terribly meaningful.
2) It's way too early to write off the candidate who's currently leading in the polls[0] in every state but two[1], as well as racked up the largest number of endorsements faster than virtually any primary candidate in history.
[0] And who's also already won the "invisible primary" (note how few candidates are running on the Democratic ticket - there's a reason most serious candidates opted out of this primary, and her name is Clinton.)
[1] and only leading in those two states when polls include a candidate who's not even running (Biden)!
Pretty much zero chance of that happening. I'll concede that Bush has had a mini-implosion but Clinton is still well on track to be nominee.
It's hard to tell, though. Polls don't work any more. Gallup recently admitted this. Not only do half of Americans not have a landline (they can only call landlines), on 9 out of 10 calls Gallup makes, the other party won't talk to them.