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Yes this is the most ideal solution, but sometimes big kids need lifts to places too, and the complexity of riding through certain areas (in london anyway!) where you might feel unsafe or anxious about a less experienced rider navigating independently makes putting them in your bike quite an attractive prospect.
Also when I had a mahoosive cargo bike id often find myself carrying my gf and son who both have their own bikes places. It’s just convenient, like having a car I guess.
I think the best solution for bigger kids is to teach them how to ride a bicycle on their own.