How did this borefest last 12 years?

Because people are lazy creatures of habit.

The show would get marginally better or worse at times but all of the problems people came to hate about it (and to attribute to whatever season alienated them from it) were present from early on--many in the 1st season and all of them by the 2nd. It's just that people got tired of them at different speeds.

Borefest to you. Lasting 11 seasons is very impressive for any show. Lasting 11 seasons implies it was very popular, just not to you. I generally don't like Westerns, but this show definitely had that vibe. There were many poignant quiet moments, but that's what made it great. Along with a ton of very memorable moments and situations. FYI: the show wasn't meant to be realistic.

I was into season 1. Season 2 was okay. I wasn't invested by season three but I watched with my wife until Glen died, season 6. The show had no hope or levity.

It's well acted and it has some good gritty elements. I decided to throw in Z Nation just to see the schlocky ripoff and I found myself really enjoying that because it had hope and a sense of humor. It's stupid, the acting is not good, but for a zombie show, it was fun.

First 4 seasons are great, 4 being the best in my view. After that, it declines.

Season 1 was okay. Everything after that was dire. I think that a lot of people kept watching in the hopes that it would get better, and it never did.

https://en.wikipedia.org/wiki/The_Walking_Dead_(TV_series)#Ratings

The first season was great, and innovative. Zombies weren't worn out back then. The next few seasons were pretty good, but then it became The Walking Dead... I mean, it just droned on and on, never dying.