The major argument against Sanders was that his platform was unrealistic. Remembering the disappointment that President Obama stoked when he was unable to execute much of his platform, many Democratic electors said they wanted the candidate that had a more modest platform that could be realistically achieved. In hindsight, sure, you are right. It's hard to know what do in the moment.
Even if you concede that Sanders' platform was unrealistic, he would have been running against a candidate that wants to build a wall costing somewhere north of 11-figures, get Mexico to pay for it, negotiate better trade deals through sheer force of will and deport millions of illegal immigrants.
It's clear the electorate didn't take reality into consideration when casting their votes.
In retrospect, it was clear that unrealistic promises being reneged mid-campaign weren't liabilities for the Trump campaign. I'm saying that at the time, it wasn't immediately apparent that Sanders's popularity would hold up in the general election.
