The Problem with Tank Ekko
I've played Ekko for quite a while, hell I remember playing him non-stop when he first came out to get his rank 5 champion mastery before he was on rotation, and I've always had people hating me for playing him. Whether it be because he just came out and I sucked (Never did him in ranked just FYI) or because he was OP before, or because I was doing too much damage for all my CC. But not once did anyone complain about me because I was simply too tanky. Sure, I did some bruiser builds, but every time I played him I focused on damage, because that is what makes the champ fun. The risk vs. reward aspect of their kit, where if you don't manage to play well you just end up doing nothing and die. Building tank items defeat the purpose.
This problem of supposedly high risk champion building tank is something that became more commonplace in recent years. Fiora is one the most prominent examples, because of her Max % HP True Damage allowing her to always be relevant in fights and tank items making her nigh invincible. Ekko's problem is the massive amounts of utility he brings, which outclasses most support champs. I've always agreed it was a bit over the top, and have supported most decisions about nerfing it, aside from massive and champion-destroying nerfs (Having his W's set up time to 6+ seconds, not give a shield, not stun).
But honestly, the problem lies in the lack of need for building as an assassin. Building survivability is understandable on any champion, but what would the win rate be of a Leblanc whose core build was this?

Notice how no damage items are needed, and Ekko's winrate is still one of the highest these past few patches. His kit is simply too good for how simple it is, and I will be one of the first to admit that. Many posts have stated that he needs to have his utility scale with AP, and while that is something that has never been seen before and would probably hurt AP Ekko's early game quite significantly, it would be the best nerf to make sure Ekko stays viable in the way he was supposed to be.
TL;DR Building a damage based champ is boring and defeats the purpose of the champ, and when it is their most successful build something drastic needs to be done ASAP.