I've read a lot of this and I'll summarize...
Lumia was not embraced by new leadership because by that time Lumia sells and Windows 10 Mobile Market Share had dropped significantly. there is a huge cost in maintaining a manufacturing process. I believe they got what they want out of it beyond patents... They did get the Windows 10 devices pushed out so that they could work on the technologies that will be used in mobile. If anything, they should consider licensing Lumia design and name to others. I just don't see anyone building a phone that will probably never come off the 1709.
The future with Windows 10 on ARM is different. That is your "Windows 10 Mobile" of the future. With that said, again why would anyone build phones that will not have a version beyond 1709? Anyone wants to build a "phone", or better put a mobile Device that can make phone call and be a computer in your pocket will do so with CURRENT builds of Windows 10, not older builds based on 1709.