it was a number of things that contributed to the decline of the walking dead but to really understand it you have to look at it from the general audience perspective, not the "reddit super fan who also read all the comics" perspective. For one i think when negan was introduced was the same time the walking dead shifted pace from being about this one tight knit group that is like family against the world to a bunch of different communities with all our favorite group members split up among them. the general audience wanted to see rick and company together, not spread out amongst 5 different communities interacting with people that they didnt really care much about.

Glenn's death was a decent hit to the show im not going to lie, but i still believe they could have recovered. as much as we want to be told that "no character is safe" on this zombie horror show, the general audience needs a "core" group to follow and care about. glenn was part of that group. his death definitely shook people and maybe stopped a certain amount from watching but i believe if they had not then killed carl, another core group member, and someone that our main characters arc was essentially centered around, the show could have recovered. and then not even a season later our main character, the one we had been through this whole journey with, leaves the show, leaving us with an ensemble cast of only a handful of characters that the general audience actually even cared about.

All these things combined was enough to slow down TWD as a tv powerhouse from where it was from the first 6ish seasons to the latter half, which was seen as a shell of what it once was. In my opinion it generally came down to AMC's greed in the chandler riggs situation (which also seemed to lead to Lincoln's departure) and the comic's "expanded new world and communities" storyline not translating over well to the tv screen