I disagree with the assertion with "that's how its designed to work" - that may be a common implementation, but its not a fundamental design of bluetooth.
And either way, if that is the design, it is critically flawed.
I have purposely attached my headphones to my watch. I start playing music from my watch. My headphones see a weak signal from my phone, which isn't playing anything and hasn't done anything other than just sit there, and switch to my phone. That is ridiculous.
The car use case is different. You are talking about a device powering up from being powered off, which is a distinct action that I as the user have taken, where in the first minute or so the car will actively "look" for paired devices. Also, I am in close proximity to my car and have a good signal strength with it (because I am in it).
My airpods shouldn't switch devices unless the signal strength is incredible (i.e. they are right next to each other), one of the other devices is actively trying to send audio, and/or there is an active user effort to try to link with the airpods from that device.