Friendly reminder that “most CO detectors use a sensor with a defined, limited lifespan, and will not work indefinitely.”
The nice part of having them hardwired is that the house is now like an office/other building: if one trips, they all trip, so the whole house is alerted. The newer ones have far fewer false alarms, too (pizza in the oven I am looking at you...).
I'd be willing to bet the NEC says "must be able to perform without mains power" for any hard wired setup.
By all means put one in your house but I think it’s a racket to have all four detectors in your house also be carbon monoxide monitors when you can just put a battery unit at the far end of the house from the combined one.
Time to visit the hardware store