The recent Star Trek film handled the loss of Leonard Nimoy with grace. (Even though Spock Prime not appearing in any future installments could be more easily explained away.) Leia simply can’t vanish without some sort of explanation.
Exactly, Spock’s arc in Beyond is specifically defined by the loss of Nimoy and for me is the heart of the movie. I’m not sure how I’d be able to deal with 9 if Luke’s arc isn’t specifically about losing Han & Leia.