You can't really mention Drupal without mentioning this: Drupal is one of the largest open source communities in the world.
This community helps build and maintain not only Drupal core but the 40,000 plus8 modules available for download.
Many contributed modules can be easily plugged in because of this community.
Some of these modules such as SEO modules, path management modules, and CK Editor enhancements are all very stable and feature-rich. With a handful of modules and configurations, you can have a very future-proofed site.
Drupal has a security team whose primary mission is to keep the core codebase secure and also stable releases of contributed modules. This is amazing as you know that there is a team of professionals that take your site and its security seriously.
Security releases come out when needed so be sure to sign up for Drupal security news or have someone to keep your site up to date for you.
Drupal Usage Numbers Speak for Themself
8At the time of writing there is 46,999 modules.