There could be various beneficial by-products from current populisms, but one in particular is overdue. That is recognition that mature democracy means 'the people' taking responsibility for their destiny (no apology for the grandeur).
That has yet to happen. Radical democrats and populists alike still see themselves as fighting for democracy, rather as though we are still in the 19th century. Let's move the argument on to working democracy as an established (but still imperfect) system. I suspect that would be a sounder way of 'completing' the democratic revolution than socialism could ever be.
In this era of nationalist populisms around the world there is an essential psychological (as well as political) question democracies have to decide: Is 'I am because we are' to be understood in the friendly way of African Ubuntu or with narrow tribal suspicion?