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I'm guessing they went that way to make more leg room and entice buyers on a stock bike. Which me being shorter, I sure do enjoy not grinding my leg on the air cleaner.
Everything is a compromise, and as long as Harley insists on hanging the air cleaner off the side of the engine where it will rub/hit the riders leg, you have to choose between rider comfort and lower restriction. I get the impression that the air cleaner on the 2017 M8 is actually narrower than the Ventilator on the earlier models, but perhaps that is just perception since I haven't actually measured them against each other. If true, it does make me wonder why they thought the previous Ventilator was a great idea for so long but then decided to go with a narrower unit in 2017.

Eventually I would expect the aftermarket to offer larger elements for the new air cleaner, just like they did for the older one. Then the customer can decide just how much he prefers comfort versus more air flow.
Jerry