I don’t know what argument will sway Sanders supporters to vote for Clinton; probably different arguments for different people. But utter dismissal and derision is definitely not the place to start.
It would have been interesting to see if Hillary could have beat Sanders in a fair election, but with everybody (The DNC, the mainstream media, even the “liberal” media) putting their thumbs on the scale for Clinton from the very beginning, we’ll never know. And she ran a very dirty primary campaign based on pandering to “people of color” and personal attacks; ignoring 55% of the Democratic party and the issues Sanders brought to the fore. That may have won her the primary, but it will also make it nearly impossible for Clinton to unite the party against Trump.
For democracy to work, elections must be free and fair so the losers can say they were heard and the majority just disagrees with them. Sanders was not heard, when TPM stopped ignoring him, it was only to report anything negative they could come up with. When there were issues within the Democratic party to be hammered out, TPM devoted most of it’s reporting to stories about Trump. Congratulations TPM, you just might have gotten Trump elected.