But the problem with that is we simply don't know if Kylo is even supposed to have an arc with Leia. This is all fan speculation or what some fans want to see. It doesn't mean that's what Rian and co have planned. I suspect Kylo's arc belongs with Rey, to be quite honest. Of course, I could be just as wrong. None of us knows what they have in mind for EP 8 and 9.
With that said, if the rumours are true that Leia is put in a coma for most of EP 8, then if they wanted to, they could have her succumb to her injuries and die, never having her wake up. Luke could be brought in to visit her and he enters the room she's in, and looks at her for a few sad, quiet moments. Then the door slides shut behind him. Some time can pass and when he emerges from the room again, he looks at Rey and co and we're all now made aware Leia didn't make it. If the rumour is true she's in a coma, it'll actually be quite easy to have Leia pass away in the film.
Or, like others have said, we can have Leia appear in video transmissions or holograms for future scenes to disguise any unused shots with a very light blend of CGI to get away with it looking current. Disney and Lucasfilm actually have quite a few options at their disposal. I feel like everyone is making it seem like there's nothing anyone can do, but that's simply not the case at all.
Personally, I don't mind either way if they keep her alive or allow her to die in either film. Because Carrie has passed away in real life, and in such a recent time before the next film, I won't be upset with any option they chose to do to be honest.