Just to be clear theres no reason you can’t make a reverse bootloader safely. Its just harder since:
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The dual boot choice is actually made before the bootloader in the mkamsboot program, which is written in ARM assembly
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Theres no way to recover from flashing a broken binary with mkamsboot
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Theres no easy way to test the binary before flashing it
So basically you have to be at least a little familiar with ARM assembly and be willing to break your player to write one. Thats not most people who use the Sandisk firmware 
