Why Thunder uninstalls the theme Thunder Admin

Problem/Motivation

During the upgrade of Thunder 6.x -> 7.x thunder_post_update_0001_upgrade_to_thunder7() uninstalls thunder_admin and seven themes, since Seven is deprecated and thunder_admin is based on Seven.

But Thunder Admin → is compatible with ^9.2 || ^10 and insteadof Seven theme we can use https://www.drupal.org/project/seven →

I also tried to find an explanation of uninstallation of these themes in the original commit, but I didn't.

So the question is why Thunder uninstalls these themes and doesn't give users an option to move to another theme later on?
Is there a specific reason?