This is the correct way to look at the issue. Gtk+2 was the last Gtk before they went off chasing CSS themes, that in the end made a majority of the interface look like vanilla milk-toast. Gtk+2 is complete. I like that view. It was an excellent toolkit when it was finished, and the code hasn't changed. The issue for "complete" packages is simply one of "does it still build?", and if not, "is it a new compiler option default change?" or similar that just needs to be updated in the build files? (most are, few are actual API/ABI breaks)
I've commented about this before, so I'll not repeat it, but simply put, there were a lot of things that Gtk+2 does, and does well, that Gtk+3 and Gtk4 don't.. (long period) So for those that have packages based on Gtk+2, the port to Gtk+3 (or Gtk4) is significant effort. The benefits are slight, to non-existent, from an application performance or look-and-feel standpoint, so asking the developers of smaller packages -- who may not be active any longer -- to invest the time to port for next to no functional benefit it is heavy lift.
Distros have had a long history of pushing adoption of release-number-next, whether ready or not, and partnering with packages like Gtk+ and gnome (and KDE for that matter, recall the release of KDE4 (.0.4a) as the default desktop for a major distro in May, 2008), in a concerted effort to malign the older version of the toolkit and sing the praises of the new as the next best thing to sliced bread. In my view, that is a "not okay"" use of distro or developer time. Clear policies by the distro (we will include only open-source projects under active upstream development), are fine. No whisper campaigns, no dubious marketing, leave that to the politicians.
But what to do with the packages that still need Gtk+2?. Considering Gtk+2 "complete" is the correct approach. Distros need not support it, but they shouldn't discourage community support for it either. Take Arch as a study. Gtk+2 was just recently removed from Arch proper, but it is still community maintained in AUR and available to anyone running arch that needs or wants it. You just have to download the AUR package, gzip -d, and then "makepkg -s" and you have your Gtk+2. This model has worked well, and to Arch's credit they have developed the AUR and provide a good community support for those wishing (or needing) to have access to "complete" packages. Arch doesn't provide it as part of the distro, it doesn't meet the active-upstream criteria, but it is still available for those that need it. After all, "choice" was the corner-stone of the open-source movement and for Linux in particular. A "forced choice" is no choice at all, it's an ultimatum.
If Debian and others could do something similar, then a lot of very good software could be saved. A little effort pays dividends.
