Looks like disappearance of single player games is a topic rather minor and uninteresting.
Once more the evolution isn't coming from evil publishers but from players. It's since some years that I see a significant evolution, more and more players playing only marginally a few single player games and in general not liking any.
But there's a logic, the autism category has increased hugely those last 10 or 20 years, many people that was normal 30 years ago are now considered to have an autism syndrome that need be cured. The frontiers of normality has evolved a lot and now a normal human is a human hyper focused on social with a phone glued firmly in his hands, if not you are sort of mad.
It's quite possible the "requirement" to use a mobile phone, and the internet evolution, contributed significantly to this evolution, there's clearly a logic between those evolution and the evolution of autism definition.
In that context you can't be surprised that the "new humans" with some phone buried deep inside their head, are more and more focused on multiplayer games only. Single player games have hard time to develop the social aspect as well than MP games.
Ok, if I can only play indie or crowd funding single player games, that's fine, it won't kill me. At reverse torture will be easier, isolate one of those new humans during one hour and his mental is broken.