The seven days had low populations and basically were AP farms for a few individuals. Players would form a group, flip the map, then switch to characters on another faction and do it again. The end-of-campaign rewards were sad and honestly not worth it. They also didn’t really drive the large scale combat that is expected of Cyrodiil.
A 7-Day Campaign would be great for smaller scale PVP. Perhaps if a new expansion added a new zone battle with an area smaller than Cyrodiil but better group action than BGs. This would allow BGs to remain single queue and a group dynamic that was desired of BGs to be implemented elsewhere.