You assume it solves nothing. But first of all Windows on ARM would make anyone who is on the fence and who doesn't really care about those apps but cares about other solutions reconsider. Secondly, anyone who does work on PCs that demands sufficiently little CPU power and who is considering upgrading to a new desktop or laptop would consider getting Win10ARM instead, because they'd be mobile. Even if they get an iPhone it would still make sense to reconsider buying a laptop depending on how they work and on just what the ARM device looks like. Thirdly, don't underestimate the tremendous amount of Windows users out there. It's a huge amount of users. Even a fraction of them considering an ARM device will be a large group of people.
Lastly, taking that into account I would then argue that rather than Facebook coding poorly for Mobile, well for iOS and Android, one presumes separately, they could code once for mobile and adapt for UWP, using Xamarin for example. That way they'd automatically increase the app user base tremendously with relatively little 'cost'. The reason they'd then be encouraged to do so is because despite you complaining about those apps there will be many others who won't and who will get the W10ARM device for other reasons, and they are now a realistic target along with everyone on Windows 10, using UWP. In other words this all comes down partially to what amount of users one can expect to see and how developers will react to it, in addition to how we view our devices. W10ARM will allow people to view their phones less and less as pure phones. Most of what we already do is NOT using the phone as a phone, but using it as a small form-factor always-connected computer.
So the "app-gap" will literally shrink by a very large amount. And the "app problem" is something I think will be alleviated to a significant degree, not because of the technology itself, but because of what I think is the inevitable adoption of it.