I don't know, what if a third mobile OS is developed and gains traction (or is just one that I prefer) but it isn't supported by the car. What if future releases of iOS or Android are incompatible with a 10-year-old version of CarPlay or Android Auto?
CarPlay and Android Auto were engineered from the start to be as agnostic to the car's hardware as possible. Your car's stereo is really just a dumb screen (i.e. just a display and input/output interface) with the phone doing most of the rendering + a few other things (i.e. providing some car instrumentation, like fuel remaining, if the manufacturer enables it) - the hardware requirements aren't really strict from a performance standpoint (minus CarPlay Ultra, and even then, that's just a tighter integration).
I'd just prefer to minimize dependencies.