Kayn Rant: Why not just let him have BOTH FORMS available after killing certain no. of champions?

So far from what I've seen, Rhaast seem to be available first a lot more than his Shadow Assassin agenda, probably cz fighting against melee enemies makes his job a lot easier to attain a form. And as a result of this, players would mostly rush the Rhaast transformation instead of having to wait another 3+ minutes for the other form to become available.

Why not just let Kayn players have their own free will to select whichever form they would like to choose, right after he killed/assisted a certain number of champions (like around 10 perhaps?)? This would probably settle the trouble of having too many Rhaast transformation, not because it's "tactical" or whatsoever, but that it becomes available first way too many times.