You completely missed my point.
The shift points are controllable by the driver. This means they are "soft" shift points, not "hard".
Yes, if you pussy-foot the throttle, a little at a time, it will probably not downshift until it's nearly to the floor. But, if you push the throttle decisively halfway to the floor it will downshift immediately.
Using your example in heavy traffic, say traffic starts moving slowly. You release the brake and depress the accelerator slightly to roll with the flow. The transmission will upshift as necessary to keep the revs low. Say you are in 3rd gear going 20 mph or less when the traffic in front all of a sudden accelerates hard. If you gently ease onto the throttle the transmission will hold the same gear and gradually build speed very slowly. If you back the throttle off almost imperceptibly and make a sharp accelerator motion to half-throttle, the transmission will quickly downshift. The shift points are controllable by the driver.
In other words, it's not only the position of the accelerator pedal that determines shift points, but also how quickly the pedal reached it's new position.