Every layer makes trade-offs. L2 solved a scalability issue with L1, but it still has issues - From privacy issues, to fees (even if low), to the need to be online 24/7. L3 makes certain trade-offs but manages to further solve some of these by offering pretty much perfect privacy, a PoC for offline usage and actually zero fees.

Just like we needed MACs and IPs on seperate layers, what ecash brings cannot be done inherently on L2.

The end goal, much like the Internet, and is what @Karnage was getting at, is that the user won't even know what layer they're interfacing with because the UI/UX abstracts it all.