Remember folks, and I'll keep harping on about this: League's dubbed in so many sodding languages that of all the things needed for a champion update, VO is the one thing that Riot pretty much has to go out of their bounds to get done. We generally (not always) think in the perspective of whatever our native language but how many languages is League in? At least fifteen if memory serves. That's a lot of VO work to do and it's gotta be done beyond the scope of Riot's offices since they've gotta get actors from across the globe to do the work.
I'm not disagreeing; I want new VO for older champions too! But I'm just trying to convey why that's way more of a difficult task than it may appear.