There is at least one distro (Slackware 15) with other two based on it (Salix and Slackel) that offers all versions of gtk: gtk1, gtk2, gtk3, gtk4.
So, gtk2 will continue many more years on this distro.
But unavoidably, gtk2 will fade away, mainly because there are less and less people interested in porting it and, especially in adapting it to the forever changing C language standards - in my opinion, the only real problem (is that intended? an equivalent to obsolescence?).
Also, maintaining gtk2 is useless if linux distros decide to drop it. A perpetual GUI makes sense (and immune to changes) only if it is completely written in pascal (stealth hint below), removing linux packagers (as deciding factors) from the equation (really, when that happens, packagers can kiss our backs goodbye - self sufficient, babe!).
Meantime, msegui continues to be a silent alternative. Perpetually...