Note that mounting /usr early, something we *already do*, is separate from actually merging /usr with /bin and /lib. Once you mount /usr early, it's rather less important whether you actually merge the file systems. While it does let you do some interesting things, I see it as more of a cleanup.
Thanks for summarizing things so clearly : the root of the problem is indeed to mount /usr early enough at init time. Then : 1) initramfs can be seen as a well known solution that I personally dislike as explained elsewhere in this thread. But I do agree upon the ROI of using such a mechanism provided it is not imposed, 2) no separate partition for / and /usr being another one. I will probably end up changing the way I install new debian system to this one now that even SSD disks are so huge compared to system requirements,
3) merged /usr proposal being a kind of UFO...But could you elaborate a bit on "mounting /usr early, something we *already do*" if you do not implicitly refer to initramfs solution.
I understand that for complex fs setup (lvm/raid/mounted networked fs...) this may require to much work to be done realistically at first install (like the solutions above by the way), but for most common cases (two ext4fs on a same disk, or separate disk but same drivers sets), I do not get what really prevents to mount /usr really early and propose that as a viable alternative to the mess (3) we have been proposed.
--eric