I don't count Nog or Winn as main cast, because THEY don't. The main cast get their names up on screen while the orchestra is playing and not being in an episode is unusual. Nog was in 47 (27%) and Winn was in 14 (8%). They are recurring characters. I have no idea how many recurring cast from B5 of TNG or DS9 have died.
Babylon 5 has definitely had some bad luck. Most shows would have a lot of cast gone 25 years later. But 8 years after wrapping, Richard Biggs and Andreas Katsulas were already gone. I didn't care much about Dr Franklin, but at that point it was already hard to imagine a sequel without at least G'Kar. When "The Lost Tales" got made, Straczynski said as much. They needed to do small stories because they'd lost people and I thought at the time, it was just 2 people, but one was pivotal.
As I go over the list, though, I find that I spoke hastily. Babylon 5 has had TERRIBLE luck. Jeff Conaway (2011) who the show could go on without but who was already established to have kept the lights on at B5 until its end, Michael O'Hare (2012) who probably couldn't have been a part of a new show anyway, Jerry Doyle (2016) who got briefly by-passed during the original series, but by Jeff Conaway..., Stephen Furst (2017) who was perhaps supposed to lead Centauri into a new kind of future, Mira Furlan (2021) who was central. To relaunch a live-action show like they love to do these days, you have to figure out how to do a story about Sheridan, Ivanova, Lennier, Londo, and Lyta. Andrea Thompson is alive, but Talia is not. And if it is set 25 years after B5, which is how long it has been, Sheridan and Londo are also dead. Storywise it is too hard. You have to move it back before "Sleeping in the Light" because you don't have enough pieces to do without Sheridan and Londo. But Londo is off running the Centauri Empire with a Drake watcher and not doing anything heroic. And not interacting with G'Kar or Vir. Lennier can come visit Sheridan, but it would be strange to do so with Sheridan's wife and Lennier's mentor off in another room. Lyta and Ivanova could maybe have some kind of interesting story... But how do you put it all together? And since they already did "Sleeping in the Light" set 20 years later, would you just be rehashing that? They might have been advised to hold that story for a reunion, but it would have turned out to be terrible advice.
I wasn't enamored with the idea of rebooting and kinda doing the story over. I don't want to watch a parallel universe with sort of the same overarching story with new actors and perhaps new characters. Straczynski has done other interesting things and I'd rather watch him do a whole new show or something set 100 years later in that universe. I never thought about doing what they are doing. It is a lot easier to recast a cartoon than a live action. They might even be able to fool us.