I frankly think a Windows 10 PC phone would not end well.
Here's what I'm thinking:
1. Cost and finding an appropriate x86 chipset.
Intel killed the Atoms that would've been used for these and alternatives are pretty pricey!
2. Value as a phone.
Opening it up to the vast amount of x86 programs is something, but they will be unusable on a display suitable for a phone. (even with a pen) And program requirements are different too, I don't use Snapchat on my PC, I rarely use Instagram on my PC, etc. I use the web browser all the time on my PC and that won't cut it when I'm on the go.
3. Windows 10's baggage
Windows 10 on the PC has a LOT of things going on behind the scenes and I'm not sure those'll adapt to a phone very well.
And by that I mean search indexing, whatever installed applications decided to start themselves at startup (I don't like messing with this on Android, I'll like it even less on a full Windows phone)
And Windows does not have means to automatically kill legacy programs when memory is low and such. People do not want to have to manage their programs like on a PC unless we're talking webOS or even Android level task management.
4. Requirements
Windows 10's minimum requirements are something like 16 GB of storage and 2 GB of RAM for the bare minimum (most of which will be used, which is not good) and on top of that add whatever data people want to add, their programs, any malware or extra programs running in the background...
And for what? I suppose we'll get damn impressive Continuum experience but it'll need to be a solid phone first on its own.
1 microsoft could easily get an atom replacement if they so desire, for that hypothetical surface phone or whatever, intel is open for business as long as you pay them
2 dont close up your mind on the notion that x86 programs means running photoshop CS something, there are "modern" apps that are x86-x64 only, if I remember correctly the skype translator beta was only x86 and that app was a carbon copy of the old app for w8 RT, as easy as can they make things, killing the x86 to ARM process for developers would make it even better
3 my anorexic atom tablet boots faster than my gaming desktop which has an NVME drive, doesn't choke on anything I could possibly do with my phone, far from it, it makes my phone look like a total waste of time for day to day usage, and thats a lumia icon which is hardly slow even today
4 using resources is good, we already have 6gb ram phones, why not keep going? vista was a resource hog they say, but if you had the resources, it ran like nothing you had ever seen at the time, and thanks to that, computers are now so overpowered that they can last you almost indefinitely unless you do gaming or video editing
and all of this I say is thanks to actually trying to replace my wp with that 8" tablet, the only problem with doing so was the poor camera and horrendous keyboard which isnt the same as wordflow on wp for some microsoft reason, all the "baggage" from w10 is actually there for a reason, its functional, like 99% of problems with w10m I could easily fix myself if it was w10, there are alternatives for everything, and you dont rely on the next update miracle that will fix whatever issue they created with the last update