Even as someone who doesn't recline much and is about 1.9 m tall and therefore in their target market, I still think that the Knee Defender should be banned from all flights. A commercial flight is a public space and one has to conduct themselves appropriately. To decide to take it upon yourself to deliberately disable a function of another passenger's chair is just selfishness. It's like those people on a crowded bus, train or tram who put their bag on the seat next to them so that they can have more room. The correct response is to sit on the bag and the correct response for Knee Defenders is to kick them off the flight and ban the passenger from the airline. I mean, where else could this lead? Someone making a product that disables AVOD screens/reading lights because the light prevents them sleeping? A barrier product for aisle passengers so that window seat passengers can't disturb them to go to the toilet?
I usually don't recline on short flights ( >3 hours) and even on long haul I don't do it all that much. That said, I think it's very unrealistic to expect that on long haul that all passengers should have their seat upright for the entire flight, especially for overnight flights. As long as the meal trays have been taken away, then I think it's pretty much open slather on such flights. If the person in from leans, then just lean yourself.