You just made the assumption that playing a video game is always unproductive, which is an entirely different argument that I find completely false but I'm not going to take the time to get into. But let it be said that not everyone that plays video games is just "wasting time" and "being unproductive", even if it is the case for you and/or you assume that it is the case for others.
Needless to say, if anyone wants to play a game that is not in its current state they typically have to resort to mods, which are also typically free to play and NOT made by the actual company themselves. From a business standpoint it would be rather impractical to cater to such a small minority, especially when it is going against their visions of where they want to take their game in genera.
My main argument is that it would be a "waste of time" in that you would either upset the masses by allowing the use of old kits in any sort of standard queue, or you would have to create a new queue specifically to allow old kits, which would likely not have many people in them and would therefore provide a likely frustrating experience for people trying to play it.
I think the only reasonable way to implement this would be to have a rotating game mode that allowed you to select from a champion's different versions of their kits. And even then I think you'd be hard pressed to get enough people into that queue to make it worth it.
