In 2016, the problem was different. Now the Democrats ran up the popular vote with lopsided victories in a handful of very liberal high-population states while falling just short of carrying a handful of more culturally conservative states in the rust belt and upper Midwest. If Trump manages to win in 2020, it will likely happen in the same way.
If the Democrats had reason to believe that this scenario would automatically be repeated forevermore, they would be justified in rejecting the country’s electoral system on the grounds of systematic bias against them. But we all know this isn’t the reality. As recently as 2008 and 2012, when the party was led by a broadly popular candidate who motivated all of its many disparate factions to show up on Election Day, the rules worked just fine, delivering solid (and in 2008, resounding) victories in the Electoral College.
What’s happened since is that the distribution of Democratic votes has become much less efficient while the distribution of Republican votes has become much more efficient. In 2016, the Republicans won just enough votes to prevail in several states they hadn’t carried in many years, while the Democrats won a handful of states (and especially very progressive California) by millions of votes. If the country had a national system for electing presidents instead of a state-based one in which electoral votes are allocated (in almost all cases) on a winner-take-all basis, the Democrats’ popular vote blow-out in 2016 would have translated into a victory. Instead, millions of votes in California and other deep-blue states were superfluous and thus effectively wasted.