I haven't read the EU proposal, but for the corresponding US rule, the motivation was to reduce interference with other radio users. In many of these products, the radio is capable of frequencies, power levels, and modulation modes that are not legal, and the device relies entirely on the firmware not choosing an illegal combination to stay legal. People have used third party firmware to allow choosing illegal radio settings.
The regulations were written in the US to be compatible with open source and customization, only requiring that outside software not be able to set the radio to transmit outside the legal limits that applied to that type of device.
A router manufacturer does not have to lock down all the firmware to satisfy this. They can make it so that the "set radio parameters" part of the firmware is separate from the rest, with only the former being restricted. This is the approach the FCC encourages, rather than simply locking down the whole thing.
In fact, when TP-Link was making routers that could operate outside legal power levels, and the FCC went after them, the consent decree they agreed to required TP-Link to work with OpenWrt and the radio chipset manufacturers to ensure that they could build compliant routers without locking out third party firmware.
The regulations were written in the US to be compatible with open source and customization, only requiring that outside software not be able to set the radio to transmit outside the legal limits that applied to that type of device.
A router manufacturer does not have to lock down all the firmware to satisfy this. They can make it so that the "set radio parameters" part of the firmware is separate from the rest, with only the former being restricted. This is the approach the FCC encourages, rather than simply locking down the whole thing.
In fact, when TP-Link was making routers that could operate outside legal power levels, and the FCC went after them, the consent decree they agreed to required TP-Link to work with OpenWrt and the radio chipset manufacturers to ensure that they could build compliant routers without locking out third party firmware.