+1 @mikeryan.
I see migrate_drupal as the successor to migrate_d2d in Drupal 7. Moreover, migrate_drupal_ui currently is only for upgrades. If the UI is generic to all migrations like the migrate_ui in Drupal 7, it would make sense to put it with migrate module (like it is in Drupal 7).
None of these modules are meant to be on in production so the usual UI vs API distinction is not so relevant.
True. And if the UI was generic enough, it would be in migrate module itself. IMHO, migrate_drupal and migrate_drupal_ui modules are not 1:1 mapped. migrate_drupal only provides plugins that are useful when the source is a Drupal database (it does not even have the migration YML files). OTOH, migrate_drupal_ui is specifically targeted around migrations tagged as 'Drupal 6' or 'Drupal 7' and running an upgrade.
+1 to leave it as it is for the current UI.