But dumping the waste cold air into the house when it's below 0C outside doesn't make much sense either.

You already dump waste hot air into your kitchen from the refrigerator during the summer. Is this much different?

It does seem a little silly to have these chains of heat pumps all working in various directions. I read about "cold district heat" in a sibling comment which circulated lukewarm water to use as a heat sink or source with heat pumps. Maybe something similar could be done with a water or refrigerant loop through the house. Probably not economical to do all the plumbing though.