In the beginning, there were arcs and there was sci-fi and they were good. But they were generally the province of science fiction literature rather than television. Until two shows came around at roughly the same time that combined both elements and set off an ongoing rivalry between the fandoms and supposedly between the showrunners themselves. I would like to set the record straight AS BEST AS I KNOW IT from having been there. I was in fandom when JMS introduced his pilot for Babylon 5. I was in fandom when Deep Space Nine premiered. I later went on to work as a writers' assistant for several years on Deep Space Nine where I had the chance to talk to the writers then and since to clarify this issue. Therefore, to the best of my knowledge and personal experience:

On Deep Space Nine, none of the writers had watched, did watch or to this day have ever watched an episode of Babylon 5.

From what I have seen, the rivalry between the two shows was predominantly promoted by the fans of each show and was not helped by JMS repeatedly publicly putting down the Trek franchise. This was his prerogative, but the folks working on Deep Space Nine that I knew really didn't care what JMS had to say. It was his opinion, so be it. Personally, I suspect the fact that JMS is so close to Harlan Ellison is possibly influential. Mr. Ellison has had a grudge against the Trek franchise (again, his right to do so) since his freelance script for the Original Star Trek Series was rewritten by staff writers. Apparently Mr. Ellison has never forgiven those who run the Trek franchise for this expected part of the life of a freelancer (being a freelancer = turning in a first draft that gets rewritten by a staff writer: that's the definition of the job!). So do the fans of JMS insist that he retains his grudge against the Trek franchise for not allowing him to create his space station show which he claimed he pitched to Paramount (I have no knowledge whether this did or did not happen).

But as to those who claim one show was 'ripped off' from the other, let me again, share some of the inner workings of TV as I've experienced them.

First, JMS did not create 'arcs'. I will refer you back to the creation of television fifty or so years ago - strongly driven by the genre of soap operas - ongoing storylines made to entice viewers (and previously, radio listeners) to return to the storylines so they'll buy more of the advertisers' soap.

Second, there was 'arc' built into the Deep Space Nine pilot: layers of characters with backgrounds and hints into their potential futures that would be expanded upon, explored and explained throughout the show's run. There were certain storylines that the writers wanted to introduce early on, such as the mystery of Odo's background. Episode six of the first season 'Captive Pursuit' featured the first contact with the Dominion, though this wasn't explained, it was IIRC what the writers intended. Other eps in that first season set up storylines that would continue for the next seven years, such as 'The Nagus', 'Battle Lines' and even 'The Forsaken'. There was always arc in Deep Space Nine, whether TPTB liked it or not, the writers did, from the beginning.

Third, here's how pitching an original story in Hollywood goes (and this is a real example - I was there):

Random Freelance Writer #1: I've got a great idea for an episode: Molly is techno-magically aged to age 16 and her parents have to cope with suddenly having a teenager! Maybe she even has a romance with Jake!

Staff Writer: Not interested. Thanks for coming in to pitch.

Random Freelance Writer #2: I've got a great idea for an episode: Molly is techno-magically aged to age 16 and her parents have to cope with suddenly having a teenager!

Staff Writer: Not interested. Thanks for coming in to pitch.

Random Freelance Writer #3: I've got a great idea for an episode: Molly is techno-magically aged to age 16 and her parents have to cope with suddenly having a teenager! Maybe she even has a romance with Jake!

Staff Writer: Not interested. Thanks for coming in to pitch.

Random Freelance Writer #4: I've got a great idea for an episode: Molly is techno-magically aged to age 16 and her parents have to cope with suddenly having a teenager!

Staff Writer: Not interested. Thanks for coming in to pitch.

Random Freelance Writer #5: I've got a great idea for an episode: Molly is techno-magically aged to age 16! See, she finds this time-thingy and falls through it and it's kind of Jungle Book-like where she's had to raise herself in the wild and her parents have to cope with suddenly having a feral teenager!

Staff Writer: Hmm, there may be something there. We'll buy that. Thanks for coming in to pitch.

That's just how it is. Completely random and unrelated people pitching the same essential ideas completely separate from each other until one hits the right take on the story and then it's bought.