Part of the "jumping the shark" was due to money craziness, and the problems when core actors decide they need to do other things with their career. The switch of captains was an enormous problem for fans and the story line, but we'd come to terms with it. The switch of first officers as well, was crippling.
The reboot of Star Trek was, admittedly, a failure. It lacked Gene Roddenberry's vision of the future as a better place as a more mature place and time with a frontier that tested and showed people who'd learned to engage frontiers with the hard-won wisdom they'd learned, who were actually making the galaxy a better place by sharing that wisdom But I was personally very pleased with the "Enterprise" series as an attempt to restart the series in an earlier period and recapture the exploration of a less mature series.
And for Star Trek/Babylon 5 comparisons, there can only be the Deep Space 9/Babylon 5 comparison. Anyone who didn't see parallels simply wasn't paying attention, and it was fascinating, as fans, to see how much better of a storyline Baboylon 5 was, and how much having a larger studio and a larger budget and franchise was able to help Deep Space 9. I really found myself wishing that Paramount, JMS, and the remainders of Gene Roddenberry's core crew and estate could have worked something out for Babylon 5 to have been told in the Star Trek universe with the larger budgets and resources.
I'm forced to admit that as a fan, I was delighted and thrilled to see Majel Barrett-Roddenberry, renowned as Gene Roddenberry's supportive wife, as Nurse Chapel and Lwaxana Troi and the voice of all the computers in Star Trek, pop up as the wife of the emperor in Babylon 5. It was wonderful to see the woman, herself, show her support of the excellent work at Babylon 5 by appear in a small bit fascinating role.
And Walter Koenig's hop from roles as Chekov in Star Trek to Alfred Bester in Babylon 5 was... well, you have to go watch the shows to understand the _completely_ different role Walter Koenig plays, and to applaud the acting and the writing that created it.