Why (out-of-universe) was this central Star Trek character killed?

Question

This central character died in Star Trek: Generations in 1994:

Captain Kirk

(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....

Answer 1

The decision was made between The Undiscovered Country and Generations, for the purpose of having a "clean" break between the original and new casts on the big screen.

William Shatner addresses the timing and rationale behind Kirk's death in his autobiography, Up Until Now (pp. 284-285):

I had been James Tiberius Kirk for almost thirty years when Paramount called me and asked if I was willing to play his death scene.

I was Jim Kirk but I didn't own the rights to me. Paramount owned the character and could do anything they wanted to him. The decision had been made by the studio that after twenty-five years the original crew of the Enterprise had finished its "five-year mission". The Star Trek movies had an average gross of about $80 million. The executives believed they might make more money with Captain Jean-Luc Picard and his Next Generation crew in command. They were determined to kill off Captain Kirk so the movie torch could be passed cleanly to Patrick Stewart's Picard. They explained their decision to me with the great sensitivity I came to expect from the studio: Kirk was going down, baby!  There was a New Generation in space. If I wanted to appear in the movie [Generations] it would be to play his death scene. But whether I agreed to appear in the movie or not, Kirk was going to die.

You may also be interested to know that:

In the early drafts of the script, Kirk took control of the Enterprise[-D] from Picard and flew it into battle against the Klingons — and died fighting for mankind at his station.

Answer 2

He wasn't

As far as I remember, Guinan spoke to Picard in the Nexus and told him that even though she had left the nexus (at the same incident when Kirk was sucked into it) a part of her (that was right now talking to Picard) was still in the nexus (as opposed to his family who were rather an imagination or projected future). We can conclude from this that a part of Kirk (and of Picard) remained in the nexus when they left. Hence Kirk lives on in the nexus. It is unclear however, if those remainders in the nexus would be able to leave it a second time (i.e., if we could have a second Guinan or another Kirk or Picard); if they would leave (and wanted to leave, which seems to be the hardest part) the nexus they could do so at any time they wished, future or past.

Answer 3

In one alternate reality, dubbed the "Shatnerverse," Kirk did survive the events of the movie. In 1996 the novel The Return was published, co-written by Shatner, Garfield Reeves-Stevens, and Judith Reeves-Stevens. In it, the Borg resurrect him through use of nanotechnology. This book was followed by Avenger, by the same writing team.