The T2 chip in the new iMac Pro is doing a lot of stuff, including offering boot protection for macOS:
⑤ Security. This new chip means storage encryption keys pass from the secure enclave to the hardware encryption engine in-chip — your key never leaves the chip. And, they it allows for hardware verification of OS, kernel, boot loader, firmware, etc. (This can be disabled…) pic.twitter.com/qKJ6bHdtr8
— Cabel Sasser (@cabel) December 12, 2017
In short, the iMac Pro has a DFU mode, not unlike iOS devices:
In certain circumstances, such as a power failure during a macOS upgrade, an iMac Pro may become unresponsive and must be restored.
To restore an iMac Pro, you need a host Mac running High Sierra and Apple Configurator 2.6, connected to the Internet. A USB cable needs to be run from the unresponsive iMac Pro to the host computer. Configurator will detect the iMac Pro and prompt you to restore and update the iBridge device — the T2 chipset — to working order. Once the update is complete, the iMac Pro will reboot into macOS.
All security measures must be weighed against the inconvenience they cause. Personally, I don’t think this tips in the wrong direction, but I know many will disagree with me.
(I assume that disabling Secure Boot doesn’t do anything to make a restore possible without a second Mac and a copy of Configurator.)
Users with bricked iMac Pros aren’t going to know how to do this, unless they are super nerdy. That may not be a big deal now, but I think it is safe to assume this sort of thing will trickle down to consumer-oriented Macs at some point. That’s not to mention the headaches this may cause in the enterprise.