Drupal as an Open Source content management system / CMF has enjoyed immense success from it's humble beginnings in 2001 in a Dorm room. The tool is now used globally for a large number of notable websites such as the Whitehouse and The Beatles.
Fueled by the capabilities of the platform and the ever growing amount of content, Drupal has also grown into big business. With Dries Buytaert the creator, leading one of the biggest. Acquia is a Drupal company with over $173.5M in funding starting from 2007. And Buytaert and Acquia are hardly alone, from the late 2000's there has certainly been a Drupal gold rush of sorts.
However looking at the adoption rate of Drupal, it is evident that Drupal usage has plateaued since early 2015. According to the official usage statistics show that up until that point the growth seemed linear, but after that it has hovered at around 1.2 Million sites:
This is probably largely due to the fact that Drupal has gone after the enterprise market and there is a much smaller number of installations it can potentially reach. Granted that these are more valuable. As a complex tool Drupal also no longer as attractive to enthusiasts and tinkerers, many of which are now opting for WordPress, Craft CMS or something completely different instead.
In terms of Dollars the Drupal industry is probably still growing, but it will be interesting to see whether Drupal really has reached it's peak in numbers has really hit is peak.