to her performance in this movie. The original plan was for her to have her largest role in Episode 9. So they were never planning on having Leia die in Last Jedi, and they said they weren't going to recast or do a CGI character. IMO it would have been disrespectful for them to have her die onscreen when it wasn't shot with that intention (I've heard some people say that they should have had her die from the explosion on the bridge.)

I think there will be a significant time jump between TLJ and Episode 9, and it will begin with them saying that Leia has passed away. Have Poe as the general in charge of the Resistance Give Rey some time to have trained with the Jedi texts she took in the Falcon and force ghost Luke, etc.