Also our immune system will be (for good reasons) killing off anything else it encounters, which means hopes like "I'm sure it'll get out-competed by $regular_bacteria in the water" are foolish.

Imagine a mirror-bacteria that does nothing directly against your cells, and seems benign at first glance... but it will slowly grow in your bloodstream and simply thicken it until your heart fails. Everyone in the world forever on artificial dialysis would not be survivable.