And no, it's not just gameplay. Design affects too.
_Edit for people ignoring this first sentence: If you don't think both things are important and affect the play rate you're very biased. The whole reason why monsters are barely played it's because of looks, regardless of how good the gameplay is._
Sure, in an ideal world, people would play monster champions just as much as attractive humanoids, but that is not the case.
I am fine with having a champion with less armor or no helm, a very minor thing to affect the game or universe, if that means that Riot will see good results and will be able to keep paying their employees to create more awesome content and keep the game alive.
If you want to show support to a champion, play it. It's very easy to talk but basically no one plays Kayle, and many things cause that. It is selfish to demand a champion staying the same when it isn't working for the company.
These outrages are often exaggerated, filled with hypocrisy and lack of understanding.