Assassins aren't even that good right now

At least competitively they aren't. Double ADC comps and utility Mage + damage Mage top (lulu/Lissandra) are way more popular. Every now and again you'll see a Leblanc or maybe even a Zed once every blue moon, but certainly it's not common.

They aren't anymore popular in solo queue now than they were 3 months ago. I think people complain about them because against disorganized teams, they are much harder to deal with so they are more pub stompish. That does not make them broken. there will always be champions who pub stomp, you can't change it. There's no sense in nerfing relatively balanced champions into the ground just to have another group of champions take their place as pub stompers and then ruin them. This then becomes a destructive nerf cycle where everything eventually sucks and the gameplay in this game becomes poop. In actuality, barring the 18 ADCs every game, the gameplay has gotten a bit better since riot has gone more to a buff up style of balance over the last year than nerf down, which is what made seasons 3/4 very chaotic and metas would shift overnight. That's probably unpopular to say but whatever, I really like the game right now. I wish ADCs would get trimmed down some, especially koggles. He's a little much.