IoT devices have a big problem with software updates. The people who make them are primarily interested in selling hardware, not supporting it for the many years it will probably be in your home. (You might replace a smartphone every two years, but how often do you replace a light switch or thermostat?)
Google says Fuchsia is built to be easy to update. Different components are able to be upgraded independently. There is a stable binary API for drivers.
Imagine a world where IoT devices are built to some Fuchsia-based standard which allows them to continue to be updated even when the manufacturer inevitably abandons the devices and their users. You could get more out of the devices, and you could worry less about security.
It wouldn't solve 100% of the problem, though, because there would always be some device-specific code, including device drivers and any software that enables unique capabilities of the device.