There are examples for both approaches. OpenBSD keeps /bin minimal and statically-linked, and it works. Solaris merged /bin and /usr/bin a long time ago, and -- unsurprisingly -- that works as well.
Frankly, as long as it doesn't bork my Debian machine when I move from Jessie to whatever Debian 9 is going to be called, they can merge all they want. At this point, seeing how many Linux systems are doing the merger, it's probably a good idea to do it, too. Packages whose developers give a flying fsck about portability (fewer and fewer nowadays because lockdown and unportability were bad only when Microsoft were doing it) already deal with both usr-merged and usr-unmerged systems (hardly rocket science in most cases), and the maintainers of packages whose developers don't give a flying fsck about portability and only develop for Linux can hope that four years from now, everyone will have merged, especially now that Debian is doing it too.
Agreed. If distros wanted to try and champion the cause for a statically linked subset of critical coreutils, they could still do it in the merged tree. The only thing the separation would facilitate would be separate mount points or physical devices.