Symfony's technical debt managment - what about you?

The terrifying mythical technical debt. How can projects like Symfony and Drupal manage to keep being relevant on the long run, when your typical apps usually need a full rewrite every few years? Everything is about processes: semantic versioning, the backward compatibility promise, the deprecation policy, the releasing workflow, a core team, a stream of contributions, a vibrant community. These are some of the key success factors of you preferred open-source softwares. Let's review how these work and take some inspiration for internal needs.