I broadly agree that the hypothesis is not firmly established and would have mild, no, or hard-to-gauge practical implications. Two quick comments though:
- The assumption that conscious subsystems don’t overlap seems unmotivated, and if it’s relaxed I think the sort of thinking that takes one in the direction of conscious subsystems (that something could be conscious while being a highly integrated component of something else) probably starts to make the individuation of subjects very difficult, producing indeterminate numbers of conscious subsystems.
- Cerebral hemispheres are an especially promising case of conscious subsystems, where no theoretical argument is needed for premise 1 (that the subsystem is capable of consciousness by itself), because people with one hemisphere removed can self-report consciousness. Worth noting that strictly, what can report consciousness is a system consisting of a hemisphere and the midbrain, etc., so drawing the inference to conscious subsystems requires accepting a degree of overlap: if the two hemisphere+midbrain systems are both conscious (as well as the whole brain) they overlap at the midbrain.