Had exactly the same experience recently. It seems like there are plenty of good mini-split heatpump systems that will work down to 0F (or even lower) without any kind of backup heat source. But if you're replacing a forced air furnace that feeds an existing ducts the only options are heatpumps that need to have backup heat under 30F. So essentially you're buying 2 furnaces in one which increases the cost. I'm in the PNW where it rarely goes below 10F so the minisplit systems would work fine without backup.
There's plenty of central split heat pumps that can function just fine below 30F. Look for ones marked "hyper heat" or advertised for use in cold climates. As long as the heatpump can handle down to 0F or so, then your backup heat only really needs to be an electric resistance heat strip (inefficient, but very cheap) since it would be used so infrequently.
OTOH, if you're replacing a gas furnace and already have A/C, then installing a new gas furnace + heat pump shouldn't cost much more than a new gas furnace + new A/C.
Imagine the electrical demand on the grid during a cold snap if everyone switched to heat pumps with resistive heating as a backup. At the time of largest demand, the largest electrical appliance in each home would be reduced to a fraction of its normal efficiency. And all the homes in the region would be experiencing that same thing at the same time.
Electric resistive heating is not a suitable backup. If adopted at scale, it would tend to amplify demand spikes when the grid is at its most vulnerable.
The ideal design (IMHO) for a cold-climate (such as New England) is forced air ducting with a heat pump (better GSHP than ASHP if possible) with a 2-zone high efficiency natural gas boiler for domestic hot water and AUX heat.
People always forget the hot water. A GSHP usually has a de-superheater that can provide some heat during the shoulder seasons, but you can't rely on it and need the backup heat (as you do for the AUX heat for both when it's super-cold out and for the defrost cycle).
A "multi position air handler" which match the aspect ratios of traditional air handler furnaces are available from the major manufacturers like Daikin or Mitsubishi.
They will pair to low ambient temperature capable condensers.
Daikin FXTQ series models
Mitsubishi SVZ series models
If you are searching.
I don't think having backup heat is a terrible idea, but it could be any fuel source. The fan should still function with minimal power to circulate air as long as there's some heat to move around.
That is what I got: a heatpump good to about 28F, and natural gas backup. I still save a lot of money over the previous 50 year old propane furnace. However so many variables changed at once I can't say if it was worth it.
