The device as a whole requires a passcode on reboot. So unless you already have the passcode, changing the Touch ID sensor won't give you access to anything. And if you do, changing the Touch ID sensor won't give you access to anything that you can't already access. (*)
(*) Some third-party apps require Touch ID auth, and if you don't disable Touch ID when a mismatched sensor is detected, it might be possible to swap Touch ID sensors and gain access to those apps using only the passcode. So it is marginally important to disable the Touch ID sensor in such a way that if the user reenables it, any previous data protected solely by Touch ID would be wiped.
But no, from a security perspective, this makes little sense unless bricking the device is the only way for them to disable Touch ID's access to the secure enclave after a certain number of authentication failures, in which case the hardware design is fundamentally wrong.
I'll give you the passcode remaining even if Touch ID is disabled. For the sake of argument let's assume Apple could do it without compromising the secure enclave, too (it simply would not be used anymore) - I don't know enough about its inner workings to say for certain.
So, Apple detects an unauthorized replacement part and disables Touch ID because they absolutely have to, as there are no guarantees the third-party component is not somehow back doored. It might allow someone to bypass fingerprint detection and by extension the passcode where it is not requested in addition to or instead of a fingerprint. They can't vouch for its security, not to their customers nor their investors.
As a result the phone is not bricked, and compared to a bricked phone that's certainly an improvement, but that's also not how people tend to think about these things. You're generally not happy because something worse could have happened, but something bad happened instead. Not dying qualifies, but we're talking about phones here.
After all, these people paid good money for a premium phone and the premium features it came with - features that likely were the reason why they upgraded or switched to that iPhone in the first place. Reality might not match up to the statement as many long-time Apple customer know, but supposedly these things
"just work". And now, all of the sudden, something in there
simply does not work.
Furthermore the device's warranty, if it had any, is negated since the phone was opened up by an unauthorized party and Apple can't know what else has gone wrong. It's resale price goes down considerably because it is no longer and never will be fully functioning. Not unless Apple agrees to do out-of-warranty repairs on these devices, replacing the innards with original parts. Potentially a more expensive operation than simply having that Touch ID sensor replaced by Apple or an authorized service point in the first place.
I have no doubt that there would be just as much bellyaching with that outcome and the people who now blame Apple for handling this badly and for being greedy would do the same then. Not just that, but instead of having broken devices properly repaired and kept in circulation in fully functional form, Apple would have outwardly trouble-free devices lacking key functionality being re-sold to people who might not be fully aware of what they're getting.
People were complaining about buying Activation Locked phones, because they were not informed. People would complain just as much about buying phones with disabled Touch ID and Apple either refusing repairs outright or asking for their out-of-warranty list price for whatever they thought needed to be replaced.
There
would be upsides - your phone breaking down in a place where you can't get authorized repairs done, or the phone breaking down when you've neglected to backup your data - but I would still not go that route. I'll rather take the peace of mind original parts and authorized service provides.
But then again I also take backups to avoid loss of important data and so far have not lost the use of my phone in a critical situation. I'm perfectly willing to admit that I might think differently if that had been my experience, and judging by replies to this thread a lot of people apparently have been there. The other option is that they're cheap and don't like the outcome of being cheap in this instance.