You hit on the problem exactly. I disagree that it takes a targeted attack; there are bots running around the web looking for vulnerabilities to exploit. Sure, a criminal won't get much from most home users but it doesn't cost much to do, and if they can get a few dozen then it would pay for itself. I've read that about 40% of home routers are configured as Internet Gateway Device (IGD). If an IGD is using UPnP, then it will accept a request to open a firewall hole from an unauthenticated Internet user. At that point it is pretty easy to install ransom-ware or do a DNS redirection and create a water-hole to get CC data. All it would take is a couple of day's work and maybe a thousand dollars to set up. The take would easily be 10 times that.
You illustrated that UPnP can be used (relatively) safely by implementing other security controls. You are doing just that by segmenting your storage and removing the sensitive data from the Internet connected devices. Just as you say that most users are not able to configure sophisticated network setups, I propose that they aren't capable of designing a security architecture as you did.
In a perfect world the vendor (such as Nuvyyo) would take all this into consideration and design a system that is both automatic (from the user's perspective) and secure. As you stated before, this is all about balancing security against functionality. I don't know how to do this with UPnP without also requiring a pretty good handle on networking technology--and if the users had the know-how to secure UPnP, then they could also manually setup port forwarding.
There are ways to do this securely. I mentioned one in an earlier post--use SSL certificates to both authenticate and encrypt the data stream. Unfortunately that means creating a PKI infrastructure just for remote viewers, or to use a commercially available structure. Both would increase cost. Perhaps there is a better, cheaper way. I haven't thought much about it, and Nuvyyo hasn't hired me to research it.
I appreciate the back and forth conversation on this. I'm glad to see that others want this company to do well. I really love the product and wish to see it succeed.