This isn't quite true ...The mainstream support is where they'd continue to add new features and software while extended support is only security fixes, while Windows 7 support ends next year that's the extended support. Its mainstream support ended back in 2015 and even then before then, they weren't adding much to Windows 7.
The situation with the drones is very different because their systems are locked down, you don't browse the internet on the Mavic, load apps etc. and the only real security risk is people hacking the Mavic's software. Therefore there isn't the same need for security updates as Windows and similar systems get as they aren't vulnerable in the same way.
Also Microsoft are drastically changing their support model to get away from long support as they had with XP and Windows 7 and are now forcing users onto much faster, more frequent changes. Even though Windows 7, 8 and 8.1 are still within their support lifecycle, you can't install them on new hardware instead you have to install Windows 10. And when you install Windows 10, the support window is far smaller so the current version 1809 which was released at the end of 2018 will reach of support next year in May 18 months later.
I don't know about US cars but in the UK recalls only happen when manufacturers are forced into either for safety reasons or because there's a major defect with the car.
I think it's likely if a major fault was found with the Mavic or there was a new hack for the software then DJI would release a new update.