I get his point, but it really depends on how you use the word "big men." Giannis, Porzingis, Davis, Cousins, KAT, Emiid, Horford, and Jokic are going nowhere. Ayton, Bagley, Jackson all look like possible upper-tier guys in this year's draft class. What about Love, Vucevic, Aldridge, Sabonis, Horford, and Gasol? What about Markannen, Collins, and Patton (I have a lot of stock in him) from last year's draft class.
That doesn't even begin to talk about rim runners like Jordan, Howard, Capella, Drummond, Kanter, Adams, Gortat, Len, Nurkic, Collins, and Favors. The 2018 draft class features possible rim runners like Bamba, Carter, Williams, Robinson, and Wiley.
I actually think there is a greater demand on the talent level for big men than ever before, or at least in the last 20 years. That demand is two-fold.
The demand is personal -- that each big man be multi-talented instead of just rim-runners. In one sense, this means that pure rim-runners are going the way of the dinosaur. If you define "big man" as a "rim runner," then it is true that it may be cut in half by 2020.
But the demand is also league-wide. For instance, part of the reason teams are going small is because they don't have the players to put two talented big men on the court together. As with any market, demand will eventually flood the market with new players.