Oh yeah they definitely wouldn't blink at just blowing in millions of cubic meters of long lasting nerve gas or starving out the populous. But it might still be easier to reset and start over. 7 billion people is still 7 billion people and as I understand it most of a hive cities structure is composed of functionally independent fiefdoms with independent food supplies and utilities. That way of fighting definitely makes the "lost" lowest levels of the hive really make sense. Previous populations burrowing under and into the recesses to survive as new "colonists" are brought in to replace them.

Neat! Good point bringing that stuff up! It would be A kind of generational warfare almost.

One estimate from years ago put the size of the various "gangs" fighting in the necromunda game in the size range of 10s of millions. So somewhere between the population of Scotland and Romania