Hey Wolfeur ,
I am the composer of the Kayn login theme(s) and thought I'd shed some light on this.
The music was designed to be a single theme, with two sides to it: the shadow assassin/Ascended Kayn side, with Ionian influences and very much acoustic-music based, and the slayer/Rhaast part, based on synthetic sounds and drums. There is a pretty fun metric modulation between the two "sides".
The music starts with shadow assassin, then goes to Rhaast's form, goes back very shortly to shadow assassin before ending with Rhaast. I thought this would be the best way to experience the music, and the form of the piece is not a hint at anything lore-related (or even gameplay related: you can only choose one path and not transform again).
Where the confusion lies is that there should have been only one login music piece uploaded to SoundCloud - the full piece, encompassing both assassin and slayer forms. This is Kayn's login theme, and should be construed as such, with both branches of his future self available there.
As to being "lazy", I am not sure where that comment is coming from :) We've been delivering a lot of great League content lately. Speaking strictly of music, you are getting two different themes as a part of a single piece of music, which, while it was the right thing to do for Kayn, is still a decent effort for a champion. The login theme is designed to exemplify two sides of evil (lawful evil and chaotic evil) in one cohesive package. As a linear piece or split into two halves, the reaction from players makes me feel like this was a successful endeavor.
Unfortunately, you can't always reconcile linear storytelling (music played from start to end) with non-linear storytelling (such as the ability to choose one form over the other, or the ability to be presented with one version of the login screen or the other at random).
Hope this helps!
Ed the Conqueror