Besides, how is this different than a gasoline-powered vehicle? In your situation, you'd still be reducing your range by X% in order to survive N hours in case something happens- your gas tank is not infinite, after all. I suppose you could make the case that you currently have more refueling options than recharging options, but I don't think it will always be that way, and it's not currently that was in some locations, such as Norway.

My gas car has a 700mi range so its simply not an issue as I tend to not drive that long on a day trip. I have the X% fuel spare for N hours of emergency heating so long as I do say < 600mi/day. I can drive indefinitely in my gas car while keeping the fuel meter over half tank and that doesn't cause any inconvenience in terms of extra stops. Doing that with an EV (never going under say 50% charge) would be inconvenient.

The same would be true for battery cars once the range is enough. And the problem of course would exist for an ICE car with a shorter range.

The heater also can't be as complex as a plug-in-hybrid, requiring expensive serveice etc. It needs to be cheap, dumb, optional (so you only waste the cargo space on extreme long journeys and in cold weather) etc.

What I'm picturing isn't a complex system. It's a camping burner with a hose to the car's ventilation system. Think < 1cubic feet and sub $1k.

Now, I am not sure how much reserve heating you want, but it’s much closer than 10x cost wise.

So a 1KW heaster for 4 hours would lose you 16 miles of range.