I found it strange that we are discussing dropping old OS support while gtk+ still has major problem on newer machines: pixelated rendering on machines with Retina displays. See for example
https://bugzilla.gnome.org/show_bug.cgi?id=710501 , there are also a lot of other similar reports on bugzilla (enter search "Retina"). Nothing done so far.
I guess users of old, outdated computers are ones that would be finding gtk+ applications useful.
I am maintaining Bluefish MacOSX port, and never was able to get good looking visuals on higher that 3.6.4 version because fo this bug.