On January 15, 2001, 20 years ago, Dries Buytaert released Drupal 1.0.0 into the world. At that time he was in his early 20’s, and just finished college. At the time, nobody would have ever thought Drupal would someday power 1 in 35 websites, and impact so many people globally.
As with anything, there are things Drupal did right, and things they could have done differently.
At the beginning of Drupal only 7% of the world's population had internet access in 2001. Smartphones or the mobile web didn't exist. Many of the largest and most prominent internet companies were either startups (e.g. Google) or had not launched yet (e.g. Facebook, Twitter). Can you still imagine?
Why has Drupal stayed relevant and thrived all these years?
Most importantly, Drupal has been focused on a problem that existed more than twenty years ago, exists today, and will exist many years from now: people and organizations need to manage content and at the same time participate on the web as efficiently as possible. Working on a long-lasting problem of this scale certainly helps you stay relevant.
Second, Drupal is made easy to adopt (which is inherent to Open Source), and kept up with the ebbs and flows of technology trends (e.g. the mobile web, being API-first, supporting multiple channels of interaction, etc).