Don't see the argument that Sanders isn't a stronger general election candidate. All it comes down to is he won't survive the Republican attack machine. If he's so fragile, why has he survived Hillary's attack machine? She has made the case he's unelectable and aggressively gone after him yet he still boasts higher favorables and better general election numbers.
"Hillary's attack machine"
You must not have been paying very much attention to this primary. Hillary has barely laid a glove on the guy.
And even if she did, the Republican hate machine would be 100x worse. Hillary doesn't have an entire media complex and an army of billionaires ready to drop hundreds of millions in fiercely negative attack ads behind her (contrary to the feverish dreams of certain people.)
Regardless, it's irrelevant at this point anyway.