Why migrate to Drupal 9:
- Drupal 9 has new superior technology.
- Drupal 9 has new superior caching.
- Drupal 7 and 8 will soon expire support.
Drupal 7 to 8 does not allow an upgrade in the same way previous versions did. Instead, Drupal 8 has builtin support for an upgrade migration. The difference from the usual website migration is that you don't build your site before importing content. Instead, you import the data into a new Drupal 8 site, and then you make the necessary changes afterwards.
The course of Drupal to Drupal 8 migration goes like this. The existing content is migrated into a new Drupal site, and then that new site with the newly migrated content undergoes additional configuration and theming, as well as some new site building, since there are some configuration, like blocks positioning, etc, that is not migrated, and needs to be manual set post migration.
Upgrade form Drupal 8 to 9 is easier than 7 to 8, but still not seamless and required developer attention.