Will this make Apple Silicon Macs a fully open platform?
No, Apple still controls the boot process and, for example, the firmware that runs on the Secure Enclave Processor. However, no modern device is “fully open” - no usable computer exists today with completely open software and hardware (as much as some companies want to market themselves as such). What ends up changing is where you draw the line between closed parts and open parts. The line on Apple Silicon Macs is when the alternate kernel image is booted, while SEP firmware remains closed - which is quite similar to the line on standard PCs, where the UEFI firmware boots the OS loader, while the ME/PSP firmware remains closed. In fact, mainstream x86 platforms are arguably more intrusive because the proprietary UEFI firmware is allowed to steal the main CPU from the OS at any time via SMM interrupts, which is not the case on Apple Silicon Macs. This has real performance/stability implications; it’s not just a philosophical issue.
And wouldn’t it be a lot cheaper to just build your own PC rather than pay the premium for the apple logo?