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It's not, in my opinion. And if the piece of software is simple and dumb, I don't really see why it should not be open source neither.

Well simple and dumb generally is good for all engineered things, as long as they aren't so dumb that they can't do their job.

This principle applies especially well to boot loaders, because the job of a bootloader is to hand control off to some other, more sophisticated piece software.

Well, you can sandbox anything that is loaded on top of a Free layer, you computer's boot firmware is certainly not on such place.