Warner talks about the decline of the "big man," but limits his analysis to centers. In reality, there are an array of guys playing power forward who would have been centers in earlier eras. Warner doesn't adequately address pace differences between eras. And he doesn't address the practice of many modern era coaches of limiting playing time for their big men.
Control for pace and playing time, and the story changes. A lot. Since 1977-78, the league has had an average of 17.2 players per season average at least 20 points and 10 rebounds per 40 minutes (standardized to a pace of 100 possessions per 48 minutes). The high-water mark for 20 and 10 players was 1997-98 when the league had 30. The low was 1987-88 when there were eight.
This season: 28.
Here's a chart showing the number of players per season who averaged at least 20 points and 10 rebounds per 40 minutes with standardized possessions (minimum 500 total minutes).
EDIT: to add a look at 20 & 10 players by number of teams. It makes sense, of course, that the total number of 20 & 10 players would increase as the size of the league increases. And that's exactly what we see. There's not a direct relationship -- meaning there wasn't a surge in 20 & 10 players each time the league expanded. But, with 30 teams, there are more opportunities for a 20 & 10 player than there are with 22 (the number of teams in the NBA in 1977-78).
Since 1977-78, the league has averaged 0.63 20 & 10 players per team. That number as increase a bit as the league has added teams.
Here's a handy table showing the movement by number of teams in the league:
20 & 10
That's about as far as I want to go with this. But, I think it's safe to say we're not seeing a decline in the quality of NBA big men.