Here's the thing, how often does that happen?
Also, your comparing apples to oranges, your not saying people in mass merge at the end of the merge lane. Then you compare it to a mass of people merging over at once. That's two totally different scenarios.
I drive on two major highways in Ohio between Dayton and NE Ohio. Bother of them go from 3 lanes to 2 lanes. No one shifts lane in mass over. Its not like they all decide, ooh lets all get over. People get over based on there driving style. Conservative drivers will get over earlier, non-conservative will get over later. Its based on a wide spectrum. Yes, at any point if a whole group of people shift over your going to have problems. It also depends on traffic flow. Since majority of all traffic studies are done at peak hours in the day, meaning highest volumes. What causes backups is just pure volume. Its like water flowing through a funnel. If you increase the flow so much that the exit rate is less than the entry rate, your going to get a back up. So if you have 3 lanes of near saturation traffic flow, and one lane drops, now 1/3rd of your traffic has to shift into a lane nearly full of its capacity. See the problem, doesn't matter were it happens, your going to get back up traffic because people will slam on there breaks, and things will slowly pass as cars slowly make there way in.
I get the theory, overall if you have people spread that merge over the entire length of the warning area. Lets say uniform distribution from 1/2 mile from merge to merge. Its easier on the other drivers, than say a ton of cars forcing there way right. Also the more down the merge your going to get more people moving to the right lane from the 2nd lane to make room as well. honestly i do this more often because i know that the 2nd lane will back up more often than not, so i ride the slow lane, it gets better flow.
But generalizing in your statement doesn't cover the total conditions or just the overall problem that your loosing 33% of your traffic capacity during peak hours.
When it comes to lighter traffic, it really doesn't matter because there suffecient room for the extra capacity. You might get some unique scenarios were a car cuts infront of another, but that can happen anywere.