Drupal 7 end of life is scheduled for Nov 2021. That date seems far but in terms of content migration could present many challenges that need to be planned right away to avoid risks.
Drupal 8 is build with Symfony, that is allowing the Drupal community to align Drupal with the latest and best practices in PHP development. The change also allows us (developers) to implement solutions from other PHP frameworks, use powerful tools like composer for package management and work with the latest and most innovative web features available in the open source community.
If it's already done, why not use it?
One of the open source advantages is the possibility to reutilize what others did into your project, moving to Drupal 8, allows Drupal to adopt most of the innovations from modern PHP frameworks.
In addition to that, Drupal 9 will allow the update from Drupal 8 without content migration, extending the expected life of the website without cost migrations. So you are migrating to Drupal 8, thinking already in Drupal 9.