True enough what you said, as this applies to most people. And I believe most people to be magnesium a deficient, and as such needs serotonin to drive bowel movement.
But with a food lifestyle that supplies one with adequate magnesium intake, bowel movement is not dependent on having a gut flora teeming with microbes that supply serotonin to drive bowel movement.
I went from being dependent on serotonin to being dependent on magnesium to drive bowel movement, and it has worked well for me.
Lately, I was prevailed upon to take a calcium channel blocker (Amlodipine) for my high blood pressure. One of the side effects is constipation, and I experienced more difficulty with my bowel movement. This is not surprising, as with the movement of calcium into and out of the cell altered , there is a change in the strong gradient between internal and external calcium ( external calcium being around 15000 more than internal calcium) which drives muscle contraction and relaxation. This would affect peristalsis, involving the gut muscles that drive bowel movement. The energy from the magnesium-ATP complex is somehow affected negatively by the calcium channel blockers, and this makes the bowel movement less natural and less effortless.