Assume Hillary wins the nomination, and subsequently gets elected, while Bernie remains a Senator until (at least) early 2019. If the Democrats don’t control both houses of Congress, then does it matter which one is President, considering that neither would get much of their legislation through Congress anyway?
Suppose the Democrats control both houses of Congress. What stops Sanders from amending a bill that comes out of the house - either, say, “A bill to establish protocol for referring to a male spouse of the President” (aka the “What do we call Bill Act”) to include a ban on college tuition, higher taxes for the wealthy, and a 0.3% tax on stock transactions (aka the “Let’s Watch Everybody’s 401(k) Crash and Burn as Nobody Is Going to Move Stocks Around Act”), or, if he wants a better chance of it becoming law, an “Equal Pay For Equal Work” or “Raise The Minimum Wage to $15/Hour” bill)? Note that he can’t introduce one himself as, if I am reading the Constitution correctly, bills that raise taxes have to start in the House.
If there isn’t support in Congress for it, then what chance would it have had had be been elected in the first place - and if there is, then would Hillary even consider vetoing it?