Except the hardware only fails to work when the software is restored or updated. Until such a time that occurs, the phone continues to work, with the third party component installed, albeit with the Touch ID functionality disabled. Some devices may never be updated to newer software, and they will continue to work without issue in this state with the only option available to the user being a passcode. It bricks however when the software is updated.
That would suggest that disabling the Touch ID sensor alone and thereby making the "secure enclave" useless is more than adequate for ensuring the phone remains secure. If it was considered a major security concern by Apple, the device would stop booting as soon as the sensor is replaced and the touch sensor and logic board keys are mismatched.
There are a lot of commenters on here that know nothing about the implications of using third party components, but also a lot of commenters that know nothing about the internal functions of the iPhone that are now claiming to be security experts based on a statement issued by Apple Public Relations, who will always issue statements that paint Apple in a favourable light because that's their sole purpose.
When it comes down to it, disabling the Touch ID and nullifying data stored within the secure space would be adequate, and Apple is capable of doing this because until now it's what they have always done. I see devices almost on a weekly basis with replaced displays and the Touch ID button replaced with a standard home button (that is NO touch functionality, that cable isn't even present or connected) that work without issue. The only difference is that the device has no Touch ID capabilities or even the option to enable it. Instead they have chosen to disable the entire device, including all of the functionality that requires no use of Touch ID, such as making calls, sending messages or using applications, when there was absolutely no need to.
That's what the issue is, and in that regard I don't believe affected owners are out of line to complain.