For Thancred himself, him dying there would have been fitting. But I think he was needed alive for Ryne's story arc if not his own. Though in a way it does advance his own with him treating her as her own person and a valued daughter figure independant of her ties to Minfilia.

Speaking of that battle...HOW did he do that? He literally can't manipulate aether right? I think he can't even use aetherytes? How did he turn invisible in a desert with no cover multiple times? I get whatever he was doing was putting a strain on him, but I was confused just as I was confused at him taking out that giant during the big ishgard tourney thing. I guess you don't need aether to do super feats of strength in the setting.