An idea to solve the issue of needing IP and Blue Essence sinks:
Allow blue essence to be converted to RP for a hefty IP cost.
Think a ratio of (at least) 500 IP per 1 Blue Essence.
This will allow players to slowly stock up RP and favor players who have less use for champion shards/blue essence/IP. Additionally, it enables players who only need a little RP for a purchase to get it on their own (which frees up Riot support originally doing it for artwork).
P.S. - No, I don't expect Riot to do this. Additionally, I fully expect whatever solution they come up with for Blue Essence/IP sink will be terrible.