This unlocks what is for me the most appealing computing fantasy: I have exactly one device to compute with. At home or work, I plug it into my dock, where it pairs to an eGPU/monitor, external storage, keyboard, and mouse, and is my desktop computer. There, I can access the 'pro' level apps we're discussing below.
On the couch or on the go, maybe I slide it into a bigger touchscreen with some extra dumb batteries, and it's now a tablet. Throw in a keyboard case, and now it's a laptop.
And if I'm sitting on the bus or walking to work, it's my phone, and I can edit today's lecture right there, or access all my files. Maybe I'm not firing up emacs or MATLAB, but it's still there if I needed to.
This, to me, is the fantasy of convergence. And considering we're now currently paying $4000+ for the trio of a laptop, tablet, and smartphone, I suspect one could create something pretty compelling and high-end and still feel like a bargain to many.
Windows 10 still contains a lot of 'tablet friendly' features, but they've been dialled down and most don't get in the way of desktop users. I think they've struck a decent balance here between the needs of desktop, netbook and tablet users, but there is nothing here for smartphone users - converging those size-constrained needs with the rest is surely the trickiest thing to get right.
When looking at creating a convergent OS, I think it's really important we look at earlier attempts, so we don't repeat past mistakes.