Just like that. Robust, simple software, open standards a r/w partition for map data. Having well-defined, narrow access to the outside world is much easier to secure and keep working.

I don't think it would. I was really thinking more about things like home appliances. An embedded computer is handy for a washing machine, for instance, but it doesn't need to be connected to the internet (no, notifying you it's finished isn't necessary: the machine should tell you when you press "start" approximately how long it will take).

For a car, I really don't know. I don't see any way of putting up-to-date navigation software into a dashboard without a significant security risk, which means needing the ability to keep software updated. Even CarPlay/AA are non-trivial and would need security updates.