But seriously, giving multiple years between the major versions is a lot of time in between for applications to port forward (or they can choose the version they want to stay on) and enough time to stabilize features that take more than 6 months to write. Turns out writing whole new rendering engines takes some time and testing and can't be flipped on as fast as javascript frameworks materialize.
If applications want to stay on a stable version for 5 years or more, that all of a sudden becomes tenable which wasn't the case before unless you wanted to stick to gtk2, which is right out of 1998 "how to write a toolkit".
This isn't very different from other systems that bump the soname and have multiple versions based on what ABI, as required by the applications. I'm sure you have multiple of these on your system already. The reason soname isn't enough for GTK has to do with parallel installability of headers, bindings, etc etc