First of all, Drupal is not dead. But I would argue it's not in healthy place relative to competing projects as it was in its heyday, in the early 2010s.
In this blog post, I will explore the problem the Drupal community finds itself in five years after a major release that broke backwards compatibility in almost every subsystem, forcing a laborious upgrade process and process shift that left many users in the dust.
I've written about this in the past, most famously in my post Drupal 8 successes and failures. I'm not going to rehash the details from that post, but I did want to focus on what I think is the primary reason for this graph's downward trajectory since 2016: