That's one disconnect I notice here; it's coming up about the TPP now but it applies more generally.

A President almost never makes a policy decision. 99% of a President's work is either administrative or ceremonial. The depressing fact is that, honestly, the only thing that matters is the letter after his or her name.

The President doesn't decide what does or doesn't go into a trade agreement, a third-level political appointee (the USTR) does (and for that matter, that's mostly his staff, who are probably careerists anyways). The President doesn't iron out deals among legislators, a third-level political appointee (a Legislative Aide) does. The President doesn't stand up to insurance companies requesting ACA premium increases, a third-level political appointee (some HHS undersecretary) does. The most practically important thing about a Presidency is which party's bag o' political hacks those third-level appointees come from, and Sanders doesn't have a different bag o' political hacks than Clinton, O'Malley, Webb, or Schweitzer (is he finally officially out of the race? That's a shame). You aren't just electing a President, you're electing a party apparatus that brings its entire inertia and baggage with it. It would be as true for Sanders as it was for Obama or W or Bill Clinton.

"Fight", "backbone", whatever: it's an Aaron Sorkin-fueled myth. Presidents allocate resources to executive agencies, and attend dinners. They find staff from the existing party infrastructure and turn them loose.