I have a hard time imagining there's much of an overlap of people who own both the Switch and Wii U, see the Switch game is full-price, then would decide to buy it digitally rather than go to their local shop or ebay to buy a physical copy for under $20 on Wii U, if price were the determining factor.
I understand Nintendo not wanting to compete with itself, but if anything, this guarantees that they get $0 from any future Wii U shopper rather than what they'd get from the few people that might buy it digitally form them on that platform.