This is basically how I've imagined it working, as well.
The release of 10.0 would shift all level 60's into level 50's, again, and all of the 9.0 (Shadowlans) content would be introduced into the pool of legacy content that players are allowed to choose to level through (i.e. BC, WotLK, Cataclysm, MoP, Warlords, Legion, BfA, and Shadowlands). Simple, clean, efficient.