You're right that most of what makes Deere tractors useful is probably proprietary, but once the base layer is open source, modifiable and reinstallable, then you can start building open source components on top that do everything else. We saw that with routers, the OpenWRT distro started out as a bunch of basic low-level code from a GPL compliance case and now there is a full-featured comprehensive web interface and a full OS for routers that is open source and runs on a variety of hardware.
https://sfconservancy.org/copyleft-compliance/firmware-liber...
It is a common misconception that GPLv2 doesn't have build/install requirements, GPLv2 does actually require users to be able to both rebuild and reinstall the GPLv2 parts.
https://sfconservancy.org/blog/2021/mar/25/install-gplv2/
https://sfconservancy.org/blog/2021/jul/23/tivoization-and-t...
https://events19.linuxfoundation.org/wp-content/uploads/2017...