what’s really federation on a system that isolates conversations per server? can you reply to a message from one server on another? maybe the main thing would be single account, because even friend lists from multiple accounts could be merged

I think you’re overestimating how much services care about retro-compatibility of clients. Try opening a YouTube video in the iPhone 6 app, you can’t, and not because the video is now incompatible, it’s because the old app has not been updated, and YouTube changed their API, so even if the software doesn’t change, services are not being provided to it. Same will happen to the Tesla app, Navigation, Netflix, everything that relies in external services to work. Sure, the car features will probably still work, but in the case of navigation, for example, even if it uses an offline database of maps and it calculates routes directly on the car’s hardware, new versions of the maps will not be available, or, routes might not be able to be calculated.

I wonder if in the future, installing your own software in the car will become illegal: given that car software can even control transmission, a car manufacturer could argue that it could compromise safety of the vehicle and pedestrians, an unsafe car framework could mean that someone can potentially program the car to accelerate when frontal sensors detect someone, to be a bit extreme, but don’t tell me its impossible.