On the contrary I think a stable cast is more important to continue the storyline and carry things over between expansions. I don't think main cast attrition between parts of a long-form story will work well to keep people engaged with what's happening and that's part of where I think FFXI's story has weaknesses.
I do think that it would be nicer if the cast had more diverse opinions and personalities, but killing them off just to introduce new cast feels like an option of last resort and will only make players resent the new characters.
From what little can be gleaned from information we have on Dawntrail, it seems there may be a schism among the Scions coming up anyway.
Fran and Balthier send the rest of the cast off while explosions are happening all around them as the Bahamut crashes but their "sacrifice" is undone right after when the Strahl is stolen back by them a year later and they show up in the sequel with no explanation with how they survived. Y'shtola at least gets healed or wooshed back into existence on screen.
Rasler dies after a minute of screentime in the intro movie. Reks is a sacrificial pawn for the tutorial prologue and we find out later in the game that he doesn't even die on-screen and wastes away while convalescing from his stab wound sometime in between the prologue and the actual start of the game.
After the first 10 minutes, and through the rest of the game for the next 40-60 hours including the end, the only character deaths of "protagonists" come from Vossler, who betrays the group just before he dies, and Reddas, who was an ex-Archadian judge who was looking to die. Neither are major characters. They are temporary guest party members on the same level as Haurchefant. If you are not a main party character, you are not a main character. If you're not important enough to show up on the box or on the poster, you are not a main character. Holding onto them to do side quests that have nothing to do with him doesn't suddenly make either one a main character. Larsa shows up on a poster but I think it would still be much too generous to call him a "main character". Penelo doesn't really deserve to be there, but she at least has the decency to stick around the whole time.