This is an excellent point. The three Kobo devices that I've owned (Touch, Vox, Aura) have a partition with the firmware as it shipped from the factory. To my knowledge, it is impossible to modify this firmware without a conscious decision of the user as well as considerable technical knowledge. While I can't speak for the Vox, since it's now packed away, the factory reset is initiated by the bootstrap so it takes even deeper technical knowledge to make a factory reset impossible. And if your knowledge is that deep, hopefully you have enough sense to create a system image first and are capable of doing the open heart surgery bit.

It is hard to break a Kobo. (Though there are times when it has made me wonder if recovery is possible, my Touch and Aura have always come through.)