Personally I think the death of open world content was lead partially by people who would sit and camp spawn points for hours or days to try and kill it before anyone else realized it had even spawned. Then there was the graveyard zerg parties for big open world bosses. Where there was no real challenge except how many warm bodies you could throw at it until it died. The greed of the playerbase, at least a portion of it, lead to them having to instance everything so everyone could at least experience it. I still see this sort of behaviour even today. The hunts in FFXIV, world rares in WoW, people rush in and zerg things down before most can react and get indignant if asked to sit back and wait for more than 5 seconds. Ive stood next to a hunt target and it was zerged down in front of my eyes before my first cast could finish.

Impatience and Greed, the killers of Open World content.