It will not be a thing because it's a bad idea.
But here is an example reason why it is a bad idea:
You exchange your extra IP for RP. You take your RP and buy IP boosts. You make tons of IP with boosts and then exchange it for RP.
This is the epitome of something for nothing.
Now, I'm not a big fan of microtransactions, but I do think there needs to be a hard divider between IP and RP purchasable content. Right now, that divider is money. Real money. Anyone can make IP just playing the game. That's the idea behind it.
Taken in the other direction, for fun, if you could convert RP directly to IP, this would also knee cap the balancing system. Yes, you can buy IP boosts with RP, but you still need to play the game to earn that boosted IP. Why do I bring this up? I'll tell you. Actually, I'll ask you.
Why do Runes cost IP and not RP?
Because they change the game by making you stronger than a player who does not have them.
If you could buy Runes with RP, the game would become what is known as pay-to-win.
inb4 you say "The game IS pay-to-win because you can buy the strongest champs with RP!"
You're not wrong, but this was also not the original intent. In a perfect world, all the champs would be of the same power level, simply offering that power in different ways. And when you have people that know their champ well...well, they can take their Urgot and wreck your Tristana.
Summary: Converting IP to RP destroys the paradigm of having certain content available only through paid transactions and nullifies whatever portion of the revenue comes from RP purchases. It is something that will not happen.
