That's a HUGE assumption to make. I don't see how that's something that can just be assumed. There is no guarantee of backwards compatibility, let alone any assurances that current purchases will carry forward into any hypothetical account system. We don't even know how any such system may work.
"In addition, we tried to encourage consumers to upgrade from an existing handheld device to a new handheld device, or from an existing console to a new console, by providing backward compatibility that enabled them to take their software assets from their existing system. However, we became disconnected with our consumers with the launch of each new device as we could only form device-based relationships."
"This is the first step of our efforts to transform customer relationship management from device-based to account-based, namely, consumer-based, through which we aim to establish long-term relationships with individual consumers, unaffected by the lifespans of our systems"
"Of course, when we do launch new hardware in the future, rather than re-creating an installed base from scratch as we did in the past, we wish to build on our existing connections with our consumers through NNIDs and continue to maintain them. "
He uses backward compatibility tied to hardware as an introduction to an account based system and specifically says they'll use the current NNIDs. The games you buy digitally are tied to the NNIDs.
Now, we don't know if the next consoles will be backwards compatible, it could be a PS4 scenario, but the purchases we make now will stay on our account and that is the problem with the lack of an account system. It's important to realize that we already have accounts, they're just tied to hardware.