Sorry youre being downvoted, but thats largely due to the body of your post complaining about a new champion instead of accepting the L and learning from it.
Yuumi herself is not a threat, don't think of her as one. she's an item more than she is a champion.
But the title is completely accurate, and your second sentence explains it. Back in 2013,Riot released Ultra Rapid Fire as an April Fool's game, but the insanely fast pace and reduced strategy/micro-managing ended up making the original game feel muddy and restrictive by comparison; So people began to lose interest. People begged for URF back, so Riot released it a few more times, but each time the same problem happened.
People began to play en masse, get addicted to URF and then quit in larger numbers once URF went away again.
Enabling it permanently would reduce the incentive to play Ranked so Riot fought for a middle ground -
Making every item give ridiculous mana and/or CDR and making refund mechanics more prevalent and easy to use leads to the game feeling faster paced and taking the worries of mana away from the game after lane phase. This, by design, URF-ifies Ranked League of Legends. And thats why damage is so high, thats why CDR is free, thats why mana means nothing outside of lane. It all stemmed from Riot's desperate attempt to make the game more like URF and attempt to regain the players who got addicted and quit, or reduce and prevent players from quitting once URF is taken down. But really, there's a ton of items with CDR that just simply don't need to have it.
Half of these either don't build out of anything with CDR (namely the first two) or could have a CDR item replaced with ... anything else.
Banshees: (partial revert)
Duskblade: (partial revert)
Twin Shadows:
The Black Cleaver:
Keep its recipe, reduce to 10% CDR.
Malmortius:
Knight's Vow:
Zhonya's: (Partial revert)
Yuumi support, and I thought she was annoying at first, but THEN she got the CDR cap and wouldn't stop firing her targeted slow and healing / shielding her partner. It never felt like I had a window of opportunity_ at all_ to deal with her or the carries she was sitting on.
