For you, yes. For the largest manufacturer of mobile phones in the world, no.
You are just a bunch of Linux zealots. Linux is nothing but a simplistic/brute force bloated piece of software, with the only real advantage of being simplistic. Works well on high spec HW (but so does Windows XP).
Anyway, tell me what kind of problems Windows Embedded Compact has with scaling up, other than including proper drivers for new HW. Then explain why this is any different for Linux.
Linux is comparable to Windows 7 (NT) in terms of RT, that is completely off the charts regarding RT but good at running tons of processes where exact timing is no issue, like on a PC.
For mobile phones, running tons of processes has limited advantage, but RT capabilities becomes increasingly important when the HW spec gets lower and simpler.
Nokia is in the need for something that also work well in the sub 200$ range to replace Symbian, and that doesn't suck the life out of the batteries in a couple of hours.
Scalable for what exactly?
Clearly any scaling of that OS is to be done with a somewhat focus on phones.
WP is a modern engine, fast, compact, reliable. Linux is the old smelly, noisy and inefficient steam engine.
Wrong. A mobile phone will never be able to fully take over for PCs.
The only success Linux has had is on Android.
The only reason for that success is Googles hard line on cutting off all the bloat.
In the embedded world as well as the PC, Linux will work, but is seldom used for anything real.