No, its not an arbitrary decision - there are some pretty decent reasons why Microsoft declined to support running 16 bit apps via NTVDM/wowexec - virtual 8086 mode isn't supported in long mode - now certainly, they could have ported the functionality from NT 4 they used to get NTVDM working on non x86 processors (it included an i486 emulator) - but to what end, realistically, how many people are still running 16-bit only software?

Another thing I'd point out - Microsoft almost backed themselves into a corner worshiping at the shrine of backwards compatibility - to the point it was difficult to move their platform or their ecosystems software forward to use more modern, more secure and more reliable methods - so unless you've been very forward looking from the start (see IBM System Z) there is a real, fundamental and painful engineering cost to maintain a line to yesterday without great sacrifices to tomorrow.

I'd argue that the PC (be it Windows, OSX or Linux) is here to stay for the foreseeable future - it may not be the platform for everyone - but for many workloads and applications the web, or mobile simply will not do.