To address the original question, I really, really really want W10M to save the Windows phone. HOWEVER, I am so disappointed by the way all the fantastic UI innovations in WP8.1 have been abandoned in W10M (yes, I KNOW it's a beta, but the direction is clear). I really dislike using it; it's like a crude copy of Android, and I'd rather use a real Android phone than this. With sorrow I've rolled back to WP8.1 and expect to stay there.
This sacrifice was necessary, apparently, in order to support the "universal app" concept, with a common user experience across all three platforms (desktop, tablet, phone). Personally I am EXTREMELY sceptical that this will work for apps with any decent functionality.
Take Outlook 2013, for instance. It's a complex desktop app with a lot of functionality and a complex UI. It would be a VAST amount of work to design a new UI for it that would kick in when the software found itself on a tablet, and yet another UI that would take over on a phone. In fact it seems inconceivable that Outlook 2013 could ever be useable on a 5" screen.
This is true of a whole range of Windows desktop applications - the idea that the developers will manage to make a reasonably functional app that will span all three platforms is for the birds, in my view. I think most of them will only ever span two (desktop/tablet, or tablet/phone). Maybe that is all it takes, but we seem to have made a big sacrifice in the UI in order to enjoy the "benefits" of universal apps. (Exactly the same is true of programs written in Java, for example: multi-platform, so a crap lowest-common-denominator UI.)
Microsoft make great play of "One experience, all platforms". But is "one experience" REALLY that important? Apple and Google have managed extremely well so far without it.
So no, I don't think W10M will save the Windows phone. I think it COULD have done, but Microsoft have blown it. I am deeply saddened by that: I wanted W10M to be a super-polished version of WP8.1, and I guess we'll never get to see that.