DS9 is a good TV series, and I enjoyed it
Btw, the originality of DS9 is no longer inside the context of our thread. We discuss about what is Star Trek, and it's future, not about the originality of DS9 concept. It is about whether DS9 should be considered as Star Trek or not. Well, it is Star Trek DS9, not Star Trek. So I leave that interpretation to everyone here.
So what is Star Trek?
I'll repeat something that I've written in another thread.
There is a franchise from Japan that has comparable in term of popularity with Star Trek. They call it Gundam. Just like Star Trek, they have a lot of series that told about a lot of different story. Some is good, the others just simply bad. Maybe many of us in this forum doesn't know about Gundam, as it is an animation series. A cartoon (although they have a live action movie). Although they are cartoon that aim older audience (older than the other Japanese Animation / cartoon), but people still consider cartoon as kid show.
Well, when Star Trek has several series in her franchise, like TOS, TNG, DS9, Voyager, and Enterprise, Gundam has a lot more series than Star Trek. Span from their original universe that they call Universal Century, to many alternate universe that has no correlation with their original universe, the UC. But it's work; and these alternate universe still considered as Gundam, and they have their own fans, although some of UC hardcore fans hate them.
So why Star Trek afraid to expand themselves to the unknown? Make an alternate universe. If it's good, they can gather new fans. For the hardcore original universe / prime universe, they still can throw some prime universe sometime. So why not? It's a win-win solution for us all.