Has a court officially ruled on this before? Until then there is no line in the sand and it's opinion vs opinion. I suspect the courts would find it difficult to prosecute somebody over this as it's so open ended (although I'm not a lawyer!).
As a thought experiment it would be interesting to see if a local area flight was prosecuted and you had nothing, whether you could argue in court that there were no applicable maps and charts and/or that you weren't flying on a route.
At the flying school I'm at (operating under AOC), the min to carry for local training area flights is an A5 photocopy of a topo map and the local approach plates for 3 airfields. No ersa, no AIP, no other maps.
When operating outside training area, need to take a few maps and ersa (plus relevant approach plates).