Don't forget to factor in the cost of maintaining the furnace, and the space it takes up in your home that could be used for other things. The running cost is only part of the total cost of ownership equation.
Usually a gas furnace doesn't take up any additional space if you have an air conditioner. It shares the fan and duct system with the air conditioner. There's an extra vent pipe up to the roof, and they're typically taller, but you're not going to store something on top of an air conditioner anyway.
As for maintenance, a gas burner is basically passive. Gas flows through a tube and comes out through holes and burns. You do have a gas valve that could fail, and you do have an electric igniter that could fail, and in theory, the heat shield could develop a hole or something, but realistically, if something fails in a furnace, it will probably be the blower, which is shared with the air conditioner. So you're not adding much new hardware that can fail.
Air quality is great and all, but when you're asking people to pay a whopping 41% premium for their heating, you're going to have a hard time justifying it by saying, "You're saving the planet."
And no, I basically have been told that I can't put solar on my house. It's a modular home, and solar companies won't touch them with a ten meter pole because of inadequate structural bracing.
Additionally, my electricity isn't metered by PG&E, which means I'm stuck on a rate plan that I can't control, and I can't put power back onto the grid, so even if I could convince someone to install it, the only option for solar would involve going off-grid with a transfer switch for when the off-grid power runs out. PG&E is in the process of running new lines (and wrecking the neighborhood in the process, and putting up ugly meters in front of our houses whether we like it or not), but that project won't be finished until probably 2027, if memory serves.
So basically, the folks making the rules have made it really, really hard to do the right thing even if you want to. Were it not for that, I'd have been on solar power ten years ago, and I'd have installed a heat pump when I replaced my air conditioner a couple of years ago. Instead, I'm seriously considering preemptively replacing my gas water heater in late 2026, because if I end up paying a 40% premium for hot water, that premature replacement would save me ~$2500 over the ten-year life of that new water heater.
But I can at least afford to do that. The poorest Californians are screwed. They can't afford solar, and they can't afford to pay half again more for hot water and heat. That's why it seriously angers me to see CARB banning gas water heaters and furnaces without doing anything to fix our obscene electricity prices.
The fact of the matter is that California's electrical prices are a major environmental disaster in the making, and have been for a long time. They are the reason why heat pumps and heat pump water heaters and EVs are often less affordable than using fossil fuels at this point. And that's really not okay, because the opposite should be true by a very large margin. If you want to know what I want to see most from politicians, it's taking a stand on our high electrical prices, because it's a big problem that needs to be fixed.