I've had various deep discussions with contributed module maintainers recently about their process to update code to Drupal 9 and one point struck me. We are so attached to "Make it ready for Drupal 9" that a key point of the message may be lost. Check out this section of the State of Drupal keynote from DrupalCon Amsterdam 2019 where Dries Buytaert showcases Johanna's relatively simple site that she prepares for the Drupal 9 upgrade entirely in Drupal 8. Notice that she does all the steps in Drupal 8 other than the final Drupal 9 upgrade itself:
This is the base principle of the process towards Drupal 9, making your Drupal 8 site better and more prepared, so the move to Drupal 9 itself at the end is a relatively small step and you got a better Drupal 8 site in the meantime. You are not jumping over the fence all at once, but in gradual steps. I thought a comparison with Drupal 6 to 7, 7 to 8 and 8 to 9 would help, since people may have assumptions or prior experiences with those, so its worth looking at how our new process compares to the two previous transitions.