You're completely right about that. However as we saw with Corbyn playing it nice doesn't prevent you from getting flak, it might even stand in the way of launching an effective counter-attack. Hysterical slander from the entirety of the liberal establishment is a given for any leftists trying to enter the mainstream.

The reason I think we need a leftist leader with some of the same qualities of Trump is that the guy lived from the flak and all the liberal pearl clutching. He used it to energise his base and build a (totally underserved) image as an outsider. The sentiments that made "drain the swamp" and "lying media" so appealing could also be activated for the left.