It’d be more interesting for an FE game to “punish” the player for keeping everyone alive. There should be a mechanical disadvantage and story and character consequences. It won’t happen because it’s contrary to the gacha-like nature it’s taken.
Earlier Fire Emblem games had a much larger cast as you were encouraged to keep playing after you lost a unit.
This meant that neccessarily these characters had very little well character to them.
FE4 is an obvious exception to this as the save system, valkyrie staff and second generation basically expected you to and gave you the tools to keep everyone alive.
To illustrate this point let me list off how many of each class you get off the top of my head.
In FE1 you get within the first 10 chapters around 10 Cavaliers, around 3 Mercenaries/Myrmidons, 4 Pegasus Knights, around 5 bow users, 3 Mages etc
Granted I may be slightly off but compare these numbers to FE8 for a second.
In that game you get 3 Cavaliers ignoring Amelia, 1 Fighter, 1 Mage ignoring Ewan, 1 Shaman, 1 Monk etc
Its quite a stark contrast.
To punish people for keeping everyone alive is retarded, just look at Shadow Dragon.
That game punishes you for keeping everyone alive by making you miss the gaiden chapters.
And I don't need to tell you how retarded that was.