How riot removes old content
Power Creep.
It's pretty straight forward. Add in content which is more influential than the old content. Design future content around the newer content. Wait until players stop playing the older content, because it's been neglected and is now relatively weak. Use it's unpopularity as justification to rework it into a design which is more aligned with the new design, or in some cases putting the once new designs in the same position as the pre-reworked designs.
The problem with this, is that by making old content irrelevant and changing the design of the new content, then you've stopped increasing the amount of content in the game, and thereby stopped growing the game. You've changed into a system of shifting what kind of content you offer.