9 years is pretty amazing.
I used to wait until the battery died, because I’m cheap. But these days, so much of a car’s functions are stored in volatile memory that is erased if power is lost (everything from engine operating parameters to making the auto-down/up windows work right) I replace it when it starts showing signs of getting weak, and I make sure constant power is maintained while it’s being swapped - your battery shop will have a power maintainer that plugs into either the OBD2 port under the steering wheel or the cigarette lighter to keep the car powered while the battery is swapped.
It wouldn’t really hurt anything to lose power, but it would be a headache. I’d have to retrain all the windows, unlock the radio, unlock the navigation system, re-program in radio and XM presets, etc etc etc. I don’t feel like spending several hours poking buttons in my car just because I tried to stretch the battery change interval too far.