I think it exists because it's a concise way to communicate "this is ending, and will end in a predetermined amount of time, but it has not yet ended" in the same manner that sunset indicates that a day is ending but has not ended yet.
I think this reply actually captures one of the problems with the term. It's super passive.
I realize this usage is becoming an official usage, but the original meaning was something that would end by design and automatically unless it was renewed. Like a law had a sunset provision if it already had an ending named at the time it was created. So the ending is built into the original plan from the beginning, it's bound to happen unless something changes.
Like... a sunset.
It's not something passive that's happened at Google though. They're saying we are going to close a service that is open, and has been expected to stay open indefinitely. It's not a thing that is just kind of happening, it's a decision that was just made and is being newly announced.
I'm a non-native speaker and sunsetting is not clear to me. I guessed what it means because I know Google and how popular Google+ is (in relative terms, I'm sure there's still hundreds of thousands of people who will lose contacts over this), but if someone had said Cloudflare is sunsetting, I'd just not have known what it meant.
And even knowing what it means, as I do now, I wouldn't say that sunsetting is any clearer in communicating that it's "going to" end than "shutting down" or "stopping" Google+. It's not as if the proposal said "ended" or "shut down" Google+, the "are" in something like "We're shutting down Google+" signals continuation to me.
