You’re playing Fire Emblem, you’re doing your best, but suddenly, two units die during your turn. One unit died because you made a mistake, the other died due to RNG. One guy who was married and had a child, and another woman who was about to S-Support with the man she loved. You’ve got replacement units who are vastly inferior to them warming the bench, so you could play through the deaths, but… The game won’t react to their deaths, not really. Those connected to the characters won’t be changed for better or worse. There will be no new support conversations where you or any other characters help characters with dead loved ones with their feelings.
Even if Chrom watches his own wife die right in front of him, and Lucina watches her mother die right in front of her all over again, the game won’t unlock any sort of consolation prize for letting Sumia die. You won’t even unlock a bonus one-shot scene with Cynthia where she cries into Lucina’s arms over losing her mother all over again. These characters won’t gain bonus stats to symbolize how mad they are at Grima and the Grimleal over this.
There isn’t a scene where the Chrom-like character and Robin-like character are taunted by the Grima-like character with visions of everyone the heroes couldn’t save. No “Fight through the Fog of Lost Souls in the Pit of Despair while fighting monsters in an escape map getting guilt-tripped every so often by the spirit of a dead ally turned against you” moment. There is no Metal Gear Solid 3 “If you killed enemies who didn’t have to die this part is different” segment.
At best, you’re playing one of the few games where a cutscene or two might slightly change depending on whether anyone died during gameplay or not. Maybe the Lord will feel worse about recklessly rushing into danger “because that’s what a true hero does” if it gets people killed. But there is no alternate ending where too many people died to honestly call this a happy ending, and no nonstandard game over where so many people die that your army’s remaining units just abandon you and your cause for trying to do a meme run where you intentionally get everybody who can be killed killed once they outlive their usefulness.
You’re solely taking a loss when you play through deaths, a loss you don’t have to take unless you’re on a self-imposed challenge like an iron man run with no resets or turn wheel shenanigans or revival staff or character-reviving plot events or optional side chapters to revive people. You’re allowing your army to get worse. You might even be able to softlock yourself. There is no gameplay reward for it later on. The story doesn’t dynamically adapt to this loss. You’re just missing out on content. Missing out on everything that character could have said or done this time around.
I think more people would want to play through deaths if the series played around more with the idea that death is not the end of one character’s story, but the beginning of effects on those connected to that character.