Saki didn't age because she probably got to master Tomiko's DNA repairing techniques.

Regarding the ending, yes, very realistic. Survival of the fittest/strongest in the long term, regardless of morals/ethics.
And even worse, thanks to one of the most evolved/smartest queerat ever: Kiromaru.

Really, Cantus humans from the past did worse than a genocide by modifying the non cantus humans DNA to turn them into queerats. It's really a cheap price to pay, having to kill every child you think could harm your community.
With their DNA engineering capabilities, they could have removed Cantus from appearing and give the death-feedback to everyone.
Or Cantus and deathfeedback for everyone, but Cantus management is too tedious, so I prefer the former.
But since they were evil from the beginning, that was not the route taken.

As for what they did to squealer, that was clearly not from an advanced and good civilization.
They've been so far as to let the poor thing in a blob state when they were not able to get enough suffering feedback. Cause I think the poor thing wasn't able to shout or react at all in his last state.



Now, since they didn't show us if they did find the stolen babies, there's hope for an OVA where the queerats use them with even more care...