The main thing you need is for the lowest-level code to be open and replaceable/patchable because it's the only part which is actually specific to the device. Windows running on Core Boot is a better place to be than custom Linux running on opaque blob, because in the first case you can pretty easily get to newer Windows, vanilla Linux or anything else you want running on Core Boot after the original version of Windows goes out of support, and you can update Core Boot, whereas the latter often can't even get you to a newer version of Linux.

Modern coreboot depends on opaque blobs on CPU (FSP/ACM on Intel) and auxiliary processors (ME/PSP), but AMD is moving in the right direction with OpenSIL host firmware. Arm devices have their own share of firmware blobs.

A decade of security updates for routers would require stable isolation between low-level device security and IoT vendor userspace. In Sphere, the business model for 10 years of paid updates was backed by hardware isolation. Anyone know why it didn't get market traction? There was a dev board, but no products shipped.