Only a vanishingly small section of the country has design temps below 0º, and even less have design temps below -10º (https://cdn-codes-pdf.iccsafe.org/bundles/document/new_docum...). Places like Boston that seem cold have design temps of 10º, Chicago is at 2º, Portland, Maine at 1º, etc. etc. (https://codes.iccsafe.org/content/IPC2018/appendix-d-degree-...)

A solution doesn't work in 100% of areas can still be the best one in the 95% of the places that do make sense!

And to your cost concerns, there's a bunch of Federal money (being deployed by the states) in the IRA to specifically pay for low-income Heat Pump upgrades.