Or, if the user is really just a mangled, burned, horrific mess, they could just elect not to use the Touch ID feature. This seems to be a great feature for those able to use it. It's probably not yet perfect, but eminently usable and will likely get better over time (including APIs for 3rd parties to leverage eventually).
AT&T should unlock the phone if you ask once it's out of contract, but as with any customer service experience, it may or may not be a simple task.
I've never heard of anyone getting linux working on an iPhone, but if you're into challenges that may be an interesting one.