I see your point. However, the chemical you are using is not comparable with co-washing vs. shampooing. You may be scrubbing less with shampoo, and more with co-washing, but the conditioner you're using to scrub with is vastly different than the shampoo you're using. It's really not comparable - even taking into consideration the amount of scrubbing one is doing. There are always exceptions, like someone who's REALLY going to town on scrubbing.
I like a good scrub - whether shampooing, co-washing, or low-pooing. I probably do it more than most, and I've never had damage co-washing long term due to split ends. Maybe it could be contributing in some shape or form to split ends that I will have down the road, but I've been doing this long enough now to notice, and it's never been the co-washing that's been damaging.
Consider also that with co-washing, your hair is likely to be more moisturized and healthy, despite your vigorous scrubbing, and long term, THAT is what will be preventing your hair ends from splitting up the shaft vs the drying factor you will get with any amount of scrubbing from the shampoo.