If they're going to mandate locking down, lock down the WiFi radio, as that's the part that uses the radio waves. The WiFi radio can be a "black box" with it own firmware, much like on cellular phones, where the cellular radio is a similar black box.
As I understand it, that is what the FCC wants to mandate. The problem is that in order to keep costs down, a lot of the wifi hardware in the routers doesn't have separate radio firmware, everything is controlled by a single system-on-chip, sort of like those old "winmodems" that didn't contain any firmware and instead offloaded everything to the CPU via their Windows driver.
So the FCC's rules locking down the radio firmware turn out to mean that manufacturers would have to lock down the entire software stack, not because that's what the FCC really wants, but because in order to save costs the radio firmware is instead done as part of the "main" firmware.