To premise this, I've been playing since season 1. I remember when reaching the cdr cap was so inefficient it almost never happened. I also remember that not every class had access to such a ridiculous amount of cdr. When I first read the tooltip for the Transcendence mastery I almost had a heart attack, but then I realized this isn't the same game I used to play. Cdr should be valuable, because abilities should ALL feel valuable and important. You shouldn't be allowed to just chuck spells at your opponent all game hoping that 2-3 of them land and you get a kill.
Cdr scales offense AND defense, but.... mostly offense. However, the problem is, the defenses (generally) that it bolsters, are extremely powerful, or abilities that SHOULD be gated by long periods of inactivity. Shield spamming is a problem that even Riot has admitted, but instead of hitting the real problem, they want to add a shield break item active. Cool, every 60 seconds I can break that Lulu shield that's up again in 3 seconds.
Cooldown Reduction is also one of the main roots of mobility creep. Riven having 4 dashes always sucked to play against, but it's much worse now that she can literally always be dashing because she has 35% cdr after her first back. That's a problem. The reason there is no early game anymore is because you have no opportunity to outlast or outplay some opponents who just steamroll you by chucking SO MANY abilities at you that you just can't fight back. It's the Jayce problem- He will always outclass you, especially early on, because he will always have more abilities than you.
TL;DR- Nerf the ability to gain cdr, regain the ability to have a laning phase and make abilities FEEL good again. The problem isn't necessarily damage (though, I will admit that damage feels really high) but that everyone is constantly applying that damage on every single misstep.
Edit: This would also leave room to *gasp* buff abilities, because they wouldn't be coming out every 2 seconds, and could have meaningful interactions and damage behind them because in the event you miss, you can and will be punished for it, or at the very least there will be more than a few seconds to react to that miss.