I my optics, computers are here to get certain jobs done. That means it is all about applications, not eye candy: bouncing icons, themes, semi-transparent windows. My real-life work desk is not transparent, and I do not use semi-transparent paper.
Producing large applications is a lot of work, so when I write a piece of (hopefully) well-designed code, I want that code to stay written. I do not want next week’s GTK+ deprecation to come along and, effectively, cause my code to bitrot. (and I really do not want to write two different pieces of code for the job: one for “old” GTK+ and one for “new” GTK+.)
Moving from GTK+ 1.x to GTK+ 2.x was painful. I do not need anything like that again. Talks about breaking API every 3-4 years and advice like “Stay up to date, adapt your application code early” (and, by implication, often) is a clear indication that keeping applications running is likely to mean spending much time cleaning up after someone with an attention span of a few years.
Maintaining code like GTK+ is not hard. Calling it hard because you want to play with some new toy is deceiving. Maintaining can be tedious, but if you do not want to maintain, please do not start writing new GTK+ code. You will surely abandon that prematurely too, so you have no business writing library code. Instead, go write a useful application: if you abandon that, I probably do not have to care.