"Chris J Dixon" wrote in message
...

I thought the justification was that if the angle is too steep,
the fluids drain too fast, leaving the solids stranded. A lower
angle was thought to keep the solids afloat and moving.

Having solids move away too fast can drag the water out of a toilet, and
other, water traps. Look at external stacks. The toilet goes outside, and
an immediate near horizontal elbow and then tees into the drain stack. The
toilet does no go directly into a tee on the stack. The elbow reduces flow.