SURG had a Drupal 7 site that needed updating (due to D7's approaching EOL). Because the site was quite old and had accumulated a lot of unnecessary code and features over time, we decided to setup a new Backdrop site and then migrate the content using Feeds. This also gave the opportunity to clean-up the structure of the site, particularly the Species/Photographs Index. A new theme was created, based on the old design but updated for better responsiveness and mobile support.
Backdrop is the perfect fit for a small, non-profit organisation like SURG. It doesn't have the complexities and additional requirements of Drupal 8+, and it has all the familiarity of Drupal 7.