I think there is a potential issue with the overall concept and dichotomy between "owning" a device to "own" your data, and using external services that must be rented to make that happen. But it sounds like a fixable marketing/communication issue. To elaborate:
Considering R&D and manufacture, it looks to me like you're offering the box itself at cost or even below. Your obvious competitor is Synology, and they have an extensible storage space concept.
So I agree with you that the meat is probably in the Apps. You're offering an email solution as the killer subscription service using somewhat questionable marketing text. But again, that can be fixed. If you were to say "okay, we're planning on offering a range of subscription apps where the core concept is to provide a public-internet service endpoint and the resulting data gets funneled directly into your home box" then you're much clearer about the value you provide and customers would have an expectation that a) their app still works if the subscription expires (it just doesn't get any new data) and b) what they're paying for in the first place. Throw in an info graph that shows the data flow!
Of course, you might have to contend with open source implementations that offer the same endpoint services as a self-hosted option BUT you could even embrace that and make the protocols open because I suspect that the convenience of your all-in-one services would trump the complexity of self hosting in most cases.
Other services/apps you could offer along the same lines could be: federated social media, video downloading and conversion, video and audio chat, collaboration and office tools, home automation, and of course cloud backup (not the paltry 128GB currently folded into the subscription package, but a separate pay-as-you-go solution with "unlimited" storage).