It is sad what Blizzard did, they buried the real Shadowlands content and resurrected the most challenging content from the last 4 expansions. Maybe it would be a bad idea to abandon Shadowlands like this, especially if you expect people to enjoy the server in it's Firestorm afterlife!
My suggestion is while Dragonflight is in closed Beta, re-open ALL dungeons for Shadowlands M+, let us have fun, even if it doesn't count towards our M+ rating, it would be fun to be able to do all dungeons.
If you do this, it will also leave an end-state of Shadowlands that will still be enjoyable to players going back to that server, or fresh new players who want to experience Shadowlands content.