I'm not sure how they would keep track of all that unless the exact same people were in the exact same raid group at all times.
I'm all for making Raids more dynamic though so that any group size can participate and have a shot at winning. Maybe all Raid bosses could have a static Defense stat and HP is determined by how big the group is. For example, Regice and Rayquaza would have the same Defense stat but they have a tough but soloable HP that increases depending on the number of people.
The challenge of raiding a Legendary would then be similar to soloing a Tier 3. Team quality over quantity