Yes. Jayce is complex and his forms are intertwined and all that, et cetera, et cetera.
But we aren't saying rework Jayce into that. We're saying a new champion that would be this new type of form switching champion, one that wouldn't require 6 unique abilities to be a complete kit, because they wouldn't have 6 unique abilities.
Yes, Jayce as a champion would not work if you took only one of his forms. But if you took the concept of his abilities of one form, those could be a new champion.
I feel like you have gotten hung up on my comment about Jayce in my first reply to you.
And as I have said in another comment: not every champion is going to have a direct counterpick. Some will require more game knowledge and mechanics to beat, rather then just an automatic advantage against them from the game lobby.
Flex picks also prevent you from selecting a good counterpick. What would you do if the enemy team picks Shen, Irelia, Gragas, and Poppy. All of those champions can be played in at least 2 different roles, and all of them overlapping in at least one. All of them require different champions and strategies to play against, and you won't know who you're going to be facing until the loading screen comes up and you see their positions.
Likewise, the two different forms of Kayn require different strategies to play against, different items to counter them, but you won't know what to do for that until halfway into the game, and by that time, you're already on the back foot because they knew what they wanted the whole time, but you didn't.