This central character died in Star Trek: Generations in 1994:
(spoilers below are not hidden)
When and why was it decided that the character would die?
As of 1994, the characters of McCoy, Spock, and Scotty were still alive in-universe. The fates of Uhura, Sulu, Chekov were unknown, which I take to mean that no definitive decision was made to have them be dead. (It's unlikely that they lived to the late 2370s, but in McCoy's case, he did so naturally, and so it can't be ruled out for Uhura, Sulu, Chekov. The point is that the creators didn't decide to say something definitive and thereby left it "open.")
Why was it necessary to kill this character? Why not have his/her fate uncertain like Uhura, Sulu, and Chekov, or why not have him be alive in the TNG time like Scotty? Scotty was preserved to the 24th Century by the transporter loop on his ship stuck to the Dyson Sphere, and Kirk was brought to the 24th Century by the energy ribbon. Kirk could have gone on to live out his last years in the 24th Century like Scotty did....