More violence is entirely possible, with the steady worsening of homicide rates in many cities probably a more important indicator than the mobs at statues or at Amazon stores. There is a lot of late-1960s naïveté at work in progressive politics right now, and the basic sympathy a majority of Americans feel for the protests and the Black Lives Matters movement might not survive a 1970s-style degradation of public safety.
But a degradation that happens fast enough to doom a candidate like Biden seems even more unlikely than the “early herd immunity” possibility.
First, because the way Americans live now means that swing voters in the suburbs are a lot more buffered from urban crime than were white-ethnic swing voters in the 1970s, and the liberal gentrifiers who feel unsafe when crime spikes in Chicago or Washington D.C. are extremely unlikely to vote for any Republican, let alone for Trump.
Second, because Trump’s own overreactions, in Lafayette Park especially, have locked in an image of him as an instigator in his own right, an arsonist in the White House whose presence there can only make matters that much worse. Maybe there is a threshold of violence where this image changes and his instigation starts to look like necessary toughness. But it’s also possible that Trump’s incapacities now extend to an inability to ever look like the law-and-order candidate, no matter how many times he tweets the phrase.