Alright, I have to put my two cents in here:
From a strictly non-historic viewpoint, I'll try to do my best to prove this from a SOCIAL point. When you look out in a crowded area - assuming it's during something that isn't an organized event - you see a multitude of people. Every day we see people from modest clothing, to provocative, to flamboyant, to religious (( I.E; Seen in Islam and Jewish practices )). Each person shows their personality in their own unique way and you can often tell a lot about a person STRICTLY from how they dress.
Now, let's take a look at some of the champions of League - specifically some enemies. Jarvan the Fourth is enemies with Urgot and Swain. Where he dresses in practical armor and helmet, making sure to use a lance to keep his target at bay, he also dresses in very striking colors to lead his men into battle (( I.E; Yellow, Red, White )). Then there's Urgot who wears no armor - aside from the potentially protective metal pieces he's constructed of thanks to Zaunites - has a wide arrangement of weaponry that he uses despite his preference to some and has very dull, dark colors showing his DEAD ((puns of damage)) and cruel personality. Then if we look at Swain, he wears limited to no armor depending on the skin you use and doesn't even USE a weapon because of Beatrice and magic. Swain however shares a similar pattern of bright colors to inspire his men but wears those colors in a fashion similar to inspiring a sense of confidence to believing in his plan versus his actions - after all, he's five steps ahead in a game of chess you already lost, who needs a second player?
Maybe I've done a bad job at explaining that, but TL;DR before I move on: Champions in the League often have different appearances in their clothes and armor based off of their personality and iconic rivalries with other champions.
Taking a look at Morgana and Kayle, they SHOULD follow the same pattern to remain consistent - since I know that Riot <3 consistency. Let's look at Morgana first to get a ground as to how Kayle should look, since this is a debate about her and not the two of them. Morgana revolves around being very liberated and being the "rebel with a cause" and is very comfortable with showing off her body in a form of self expression and freedom - even having dark wings and colors showing off her training in dark magic. I would expect her to even choose to not wear armor - even if it would be smarter - strictly to spite those who DO wear it. Kayle, however, is modest, she intends to remove all things that could cause chaos (( after all, she believes in strict rules and order )). This means she becomes someone who is not defined by their gender, no defined by their social rank, and not defined by individuality. She becomes someone defined by her actions in accordance to her morals. That's why people are upset, because even something like "boob armor" seems unlike her and to others it seems impractical. It simply doesn't fit what she stands for to them, and people want to make sure that is made clear to those at Riot to keep her the way she is already.
I hope I did a good job at explaining that, after all, I enjoy the diversity of characters in League from J4 to Ahri to Kayle and seeing their uniqueness taken away seems wrong and out of place.
