The backup heat source of a heat pump is extremely nice to have if your furnace craps out, just shut off the gas supply and let the heat pump do its job. If I had to replace my A/C condenser I’d probably opt for a heat pump to replace it so that I’d have that option too.

    > Gas is surprisingly, weirdly cheap, often not even metered in apartments.

Yikes. How is this possible? My guess: You are paying for it in a building fee.

Generally yes. But also it is sometimes like half the cost or less for comparative heating power, and for air-heating an apartment in the middle simply doesn't need anywhere near as much, and you literally heat each other so actually-fair costs are fuzzier. Gas bills in places that didn't include it would often be like $15/month in the middle of winter, hardly worth the administrative and hardware costs.

One of the houses I rented many years ago didn't even have metered gas. We paid a "hookup" fee to the city (like... $40/quarter?), and they didn't care how much we used, as long as it wasn't for commercial purposes. I assume the city had something for detecting extreme use or leaks, but that's probably it. It was a somewhat strange property though.

> and you literally heat each other so actually-fair costs are fuzzier

What you're describing is closest to "actually fair" I think we can get. A neighbour of someone I know leaves their apartment empty for long periods of time and when they're gone they just shut off their heating completely and leave all of their neighbours to pick up the bill - each apartment has their own heating system but they share walls, floors, and ceilings.