Should they? That's a very difficult question to ask.

The primary reason they were "killed off," I think, was to break ties to the 1.x - 2.x cast and give the 3.0 cast a chance to shine. Without the Scions to guide us we had to struggle and suffer to find our way. If we had them around still, it'd just be like 2.x with Minfilia yelling "Go do X because reasons, Warrior of Light!"

(In theory. In reality, we just took orders from Alphinaud instead of Minfilia.)

Do I think dead NPCs should stay dead...? Of course I do; the resolution of the Ul'dahn coup was one of the weakest parts of the 3.0 story, in my opinion. Yes, we never personally checked Nanamo's vitals, but when someone looks like they're choking and then collapses, I'm not inclined to believe that was a [kupo!] sleeping drug. Plus, if Lolorito really was more concerned with preserving the Ul'dahn social and political systems, why did he sic the Brass Blades and Crystal Braves on us after his plan had already gone off without a hitch?! (In fact, Teledji unwittingly helped him with it!)

...

... personal grievances aside, the ultimate fate of the Scions remains inconclusive. I'd wager Yda and Papalymo somehow snuck out of Ul'dah. Thancred got warped with Y'shtola's Flow, so nobody knows where he is or if he's even on the physical plane. Minfilia ran back into a collapsed tunnel...

... /sigh.