I agree with all your major points, well said. I will only add that back in the late 90s, MMOs started to become more popular among PC gamers, and that those were definitely designed for mild addiction (to keep players paying a monthly fee).
After WoW took MMOs mainstream (by around 2010-2012 when its playerbase peaked), I feel that lines up perfectly with your observation that developers began incorporating more and more positive feedback loops into games. I only bring this up since I wonder if there’s an actual correlation there (along with the other elements you pointed out regarding accessibility, etc.) or if it’s just coincidental timing.