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.
Survived? He's losing. You know that right?
I mean her attacks on him, which include attacks on him from the right, hitting him on spending and being a socialist haven't left him with weaker favorability than her or a worse performance with Independent voters or weaker general election numbers. I'm skeptical the GOP would be able to destroy Sanders for advocating single payer when their own frontrunner has proposed the same thing and it didn't hurt him at all.
As far as the Democratic primary is concerned, Clinton started out with such a massive lead and Sanders has continued to close the gap despite her attacks. She is still winning and will be the nominee but that doesn't suggest her attacks on him have landed. Just that she had a big enough lead that she won't blow it.