I think the death toll of named characters for 4 years (earth time) of constant war with primals, garleans and dragons is actually at a reasonable level.
Moenbryda was a very short lived plot device used to give us an Ascian killing tool, show us that even Urianger can get embarrassed, and then do a lame asspull with "oh look, she left us a second crystal without saying anything, let's kill another Ascian".
Haurchefant went from minor side character to Best Bro Forever. As sad as his death was, it allowed for a tremendous amount of character growth in numerous others, and we may see those effects grow even greater in the future.
Ysayle had a splendid redemption arc spanning half the expansion, sad to see her go but I really can't imagine what else they could have done with her character by leaving her alive. Join the Scions? Start an orphanage for heretics? It's technically still a bad thing for Hydaelen every time she transforms.
Hydrus was a pure villain and his death would have been on our hands eventually anyways. Maybe we could sympathize with him for being the only humane Garlean officer ever, but his motives were (without question) to screw us over in the long run.
That last guy to recently die ..... I never saw a corpse, and this is SE we're talking about. Anything's possible.