That would require them to make a particle effect that would cover every champion in the game, and make the model disappear on death, the problem being
Is a guy who exists and becomes so big that he just becomes a lane, so the particle effect he would make (if we assume riot can find a way to make it scale to the model size of the champion) would be absolutely humongous, and depending on how long it would be could actually mess up everyone due to not being able to see what is happening, while this sounds like a cool idea, and more than likely someone at riot came up with a similar idea while they were making him, I just wanna point out the difficulties, and problems that would come with that idea, more than likely being the reason that riot didn't do it.
But i must admit that it would probably be absurdly satisfying.