Killing a whole bunch of characters is not a good idea. First they have a short time to make everything so now they'd have to make a bunch of new vendors give them possibly hundreds of voice lines in a short period of time, make stories for them, explain who the hell they are and why they're replacing these characters, introduce new characters (Devrim is married so now you'd also have to introduce a sob story which would likely mean episode 1 2 or 3 would now be a sob story rather than finishing off loose ends which would even worse for the stories than they already are), etc, etc. Arguably the better choice would've been to have an entire piece of the last city stay crystallized and those in that entire area permanently frozen even after the Final Shape this would show the stakes better while also having to not basically spend time they don't have to try and make new characters to replace old ones.