The Apple Watch is a reasonable “model” for an iPhone Nano - particularly potential functionality and battery life. The big advantage of the watch is that it is hard to lose if strapped to your wrist or charging by the bedside. I think an iPhone Nano would be too easy to lose unless worn like a fob watch or necklace.
My AW3 does not have an alpha-numeric keypad of course - which could be a nuisance with an iPhone Nano.
BTW - Siri on Homepod turned out to be helpful when an iPhone was mislaid in our house. I just asked “Where is XXX’s iPhone” and he/it replied “Nearby” and got it to sound a tone.