The real reason why black domains are silly

SPOILERS: Everyone says the rationale for the black domain is it takes too much resources to destroy a target that is “not a threat” while exposing your own position in the dark forest

Did we all forgot Cheng Xin and her boyfriend LEFT their own black domain through an alternate universe door conveniently left behind by Yun? If civilizations have the ability to leave behind doors inside a black domain, and can re enter the “great universe” at a different spot, well voila, you can have an intelligent specifies literally pop up in your backyard at any point.

So it IS economical imo to destroy them if it is NOT an immense drain on your resources. For singers species, the fact is, a 2D foil could basically be chucked anytime anywhere. This shows me certain species can destroy others very economically and without being a major resource drain. Curious to hear your counter points

Why would you risk leaving behind a species that can pop into any other spot in the universe, colonize and potentially be a threat to you?