Three options for Leia, and why re-casting is the best one
(Note: This post assumes there weren't already plans to kill off the Leia character in episode VIII.)
Option 1: Write the character out of the story, either by killing her off, or by reducing her character and moving her off-screen.
The problem with this option, if reports are to be believed, is that they already planned a big story arc for Leia through Episode IX, and this would necessitate massive rewrites that would likely hamper the story, and result in a lesser overall experience for the trilogy.
Option 2: Make Leia into a CGI character.
The problem here is that the CGI characters in Rogue One have garnered mixed reactions from fans, with many feeling that the characters failed to successfully cross the "uncanny valley". Anyhow Lucasfilm appears to have nixed this idea, releasing a statement that they have no plans to replace Carrie Fisher with a CGI representation.
Option 3: Recast the character, allowing another actress to play Leia in Episode IX.
I know many fans do not want to see another actress play Leia, but this option would allow Lucasfilm to stick with the pre-planned story arc, protecting the integrity of the original plans for the trilogy while avoiding either uncanny CGI, a dubious off-screen death, or a situation where we only hear Leia's voice (either through cleverly edited audio samples or a sound-alike voice actor) off-screen, e.g. over a radio.
Not only do I think this option is the best of the three, but if certain reports are true there is actually a good possibility that Lucasfilm is seriously considering this as an option going forward. There was a report that Lucasfilm was talking to the family of Carrie Fisher concerning her role in a future film (presumably Episode IX). With Lucasfilm announcing that there are no plans for a CGI Leia, this could mean they were asking for permission to recast the roll in hopes of avoiding any hard feelings with the family if they (i.e. Lucasfilm) should do so.