Nadella was saying that the current market share is unsustainable, not that WM was unsustainable.
As the others suggested, WM will be around for at least a few more years while Microsoft tries to break into the very lucrative smart phone market, which is where Apple makes the lion's share of their profits.
The OS is going to continue to work for past devices, even as new devices come out. With that said, I ended up returning my Lumia 950 because it was too buggy, had poor battery life, and it crashed frequently. This was days before the firmware update, which hopefully fixed a lot of my [reported] issues, but there were
a lot of problems to fix. From my perspective as a huge fan of the ecosystem, the Lumia 950 must have been a last ditch effort from a hodgepodge of employees that were [likely] partially laid off in the last big round of layoffs, followed by Microsoft's desire to keep WM somewhat relevant by releasing a new phone.
I expect that now that the phone development has been placed under the Surface team, the whole experience should be a lot better (perhaps minus display drivers

). Personally, I was really looking forward to the Lumia 950 and I was really disappointed with all of its issues; when it worked, I loved the device though, so I am looking forward to the next release.