The thing helping the Raspberry Pi fend off clones is their deal with Broadcom, not whether the board, firmware, and boot code are open or closed source. It's easy to copy the board and binary blobs either way. The closed nature of the boot code hurts tinkering without helping sales.
It's true that Broadcom is the reason why most clones are not as good as the Raspberry Pi but it is not because of the boot code. It's because Broadcom has a custom graphics chip that has nothing in common with the official ARM GPUs. Any effort into open sourcing the very popular Raspberry Pi will not help open sourcing other boards that have the way more common Mali GPUs. Raspberry Pis are their own island so to speak.
