Participation in old expansion queues can be pretty low. And if you try to queue for a dungeon that’s only available from level 25 you can wait for hours without end because people will just be snatched into a lower level dungeon. So instead of only utilizing maybe 20-25% of each expansion’s dungeons most of the time, people should just have access to all the dungeons from level 10.
It’s happening with newer expansions anyway (Legion and onwards), and it would also seem to solve the bug that makes level 51+ only access level 10 dungeons that Blizzard seems to be unable to fix.
Halls of Reflection also has a wave encounter but I’ve never actually done it in a group of appropriately leveled characters (hence the OP) so I don’t know how bad it really is.
Is Trial of the Champion considered good or bad? It has a gimmicky joust fight if I’m not mistaken (which was a pain to solo) but otherwise is just a boss attack dungeon with no trash.