Look at the controversy over the phone not booting with third-party fingerprint reader repairs as an example. People were upset when they found out that having their device worked on could make it unbootable, but Apple was able to easily fix it with a software update. If it had been designed more securely, it might have wiped data when it detected unauthorized modifications, which would have meant even more upset people. Now that this has become a public debate, there will be a very different response to making it more secure.
Making the DFU update path more complex increases the risk of bugs and thus the risk of permanently bricking phones.
You could imagine an alternative where on boot the Secure Enclave runs some code from ROM which checks that a hash of the SE firmware matches a previously signed hash, which is only updated by the Secure Enclave if the user entered their pin during the update. If it doesn't match, either wipe the device or don't boot until the previous firmware is restored.
This way Secure Enclave firmware updates and updates via DFU are still possible, but not together without wiping the device.
Honestly, this is really the shit..
That basically happened (at a smaller scale) just last week. When Apple apologized and fixed the "can't use iPhone if it's been repaired by a 3rd party" thing, the fix required updating phones which were otherwise bricked. It's not an unreasonable scenario.