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the choice of distribution in dom0 matters little today and will matter even less in the future
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dom0 is always offline / no remote attack surfaces
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as more and more hardware interfacing is moved into dedicated system qubes (sys-net, sys-usbm sys-audio, sys-gui etc.) the tasks for dom0 become less and less
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at some distant point in the future dom0 might be a Qubes OS specific minimal distribution
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Short of real and major security benefits in moving dom0 to another distribution it is simply not important enough at the moment to distract from other more urgent development tasks.