No matter how many teams, no matter how many boards at some point you are going to need one, masterful backlog to rule them all.
You’ll need a roll-up, THE roll-up that will signify what everyone in your team is doing.
This view isn’t for everyone and it won’t be broken out by sprints (they might be listed and each group might have its own) but invariably they will be listed by resource.
And that’s the role of the “Big Bad Backlog for All” – what is everyone on my team doing across their project so I can make sure I’m supporting them and giving them everything they need to be successful.