Why would Apple do that?

If they did, I still have macOS, an OS I can easily disable all runtime protections and security on, rig up into a kernel debugger, arbitrarily dump memory of other processes and so on. If Apple takes away our ability to easily boot alternative kernels, the tools are readily available to find...alternative ways around iBoot security, which is not ideal for Apple since iOS iBoot is mostly the same as it is on macOS.

I find it hard to believe that Apple would purposefully shoot themselves in their own feet, unless you also believe that they would lock down the Mac as much as an iPad, ever.