What an exceptionally moronic thing to ban, the market solves this naturally. Resistance heaters are 100% efficient whatever fraction of the year is heating days. So if that's 1/2 the year and the water heater can't last 16yr because of water quality the heat pump heater will never pay you back.
This reminds me a lot of the time some jerks in west coast desert states convinced the feds to regulate plumbing fixtures so that eastern "we take from the river and put back in the river" municipalities that have more water than they know what to do with have to suffer through low flow everything.
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.