Alright, I'm going to answer the question. I hope you actually listen instead of just behaving like your question was rhetorical all along.
The reason why Sona is getting nerfed swiftly and other champions are not is because with Sona the problem is very easy to identify. Sona is dealing too much damage with Summon: Aery. This has a lot to do with the reliability of her kit. Sona has always been a numbers kind of champion, and her numbers before the rune rework were tuned to basically make her appropriate to play against for most players. However, Summon: Aery has simply added damage to her kit, so she needs to have that damage lowered. This is why I am in general against the idea of base damage being in the game outside of kits (items, runes, etc) because then you have to balance around that damage combined with the kit and if the player chooses not to use the item or rune they end up with less damage than they are "supposed" to have.
So why doesn't this apply to the other champs? Let's take a look:
- This one is tricky. Leona's buffs a while back were meant to help her actually successfully all-in champions who were out sustaining her with Windspeaker's Blessing. The removal of Windspeaker's from the game has impacted the early game of shielding champions a lot more than anyone was expecting, and Leona's bases were tuned around the existence of MR runes but not around Leona running penetration marks. So when everyone else who deals magic damage early lost penetration, Leona didn't, and everyone around her lost MR.
She's not entirely out of line, it's just been quite a while since she was strong and people have forgotten how to play against her. I do think she's dealing too much damage, but it's not the same as with Sona because with Leona you have to go in, commit, you can miss, etc.
- The problem with Ezreal is not something that a nerf can fix. Ezreal is able to deal on-hit effects with his Mystic Shot. That is necessary for his balance, it is a huge part of his character. You can't have him deal 50% on-hit or 75% on-hit, it's either all or nothing. (Riot has recently introduced the idea of partial on-hit effects with Urgot's W, but I have no idea how that coding actually works or what it would look like with the Spellblade ability).
So Ezreal gets more gold early, he hits his power spike earlier, and then he is able to build more items quickly than other champions. But he's giving up damage to do this. Ezreal does not have Press the Attack, he does not have Electrocute, he does not have Summon: Aery or Lethal Tempo. So it's a fair trade. Honestly, considering the champion basically has 3 abilities instead of 4, this is an appropriate power level for him when he is played well. But more importantly, how do you even nerf this? Do you nerf the gold on Kleptomancy? That reduces the effectiveness of it for other champions. Do you increase the cost of Trinity Force? That hurts everyone else. Do you increase the cost of Manamune? Ezreal will build something different. It's not something that's easy to fix like "too much damage".
- I feel your pain here, this one honestly feels like total bull shit. I was level 3 at 10 minutes in a game as a support against MF. I got fucking murdered by her every time I got close. I don't know why Riot is dragging their feet on this one, they were probably waiting to see if she would be more balanced after the Aery nerfs. It's probably about time for them to do something about her.
Assassins killing everyone quickly - That's their job. Literally days before the preseason people were bitching about marksmen being immortal. Now they're not. This is good for the game.