adults dont want hardcore games, they want to make time fly when idle situations occur
I think this is it. Back in the day, there were less choices. We could afford to spend all of our time on one overly complicated game. We had few games to play and when we did, we made it count.
Now some of us who remember the "good old days", don't have the same kind of time in our adult life. some of us *gasp* may even have families now and we can't just sit in front of the computer for hours everyday. And there are others, like me, who seem to get bored quicker of the games I come across and need to change it up more often. I'd rather experience new things, than to try and be the best Dota 2 player of all time, or to explore every freakin possible skill tree path in Path of Exile. Being an adult with a full time job, gives me the ability to buy more games than I could when I was a teenager. I'll play these games a while, when they are good I make it to about 100+ hours and move on.
Every game shouldn't have to feel like you're practicing rocket surgery. It's ok for a game to be simple AND fun. People just throw this "dumb-down" label on some games to make people feel bad for liking it, when it doesn't always deserve it. Especially when some people equate Complexity with counter-intuitiveness.
Okay that's a good point.
So, 10-15 years ago, when window is something that stuck on the wall and elephant still reproduce by laying eggs, the proportion of gamers are unlike today.
At those golden age, potential target for gaming industry are majorly kids and teenager, at those time, adult that spend their time playing games are frowned upon by society, and are actually the minority of gamers. So most gamers at that time are full time gamer, that spend all their leisure time playing games and their school time dreaming about games.
Fast forward 10-15 years, Those full time gamers are now have grown into adult and some percentage of them are still so obsessive with game that despite having spouse and raising junior gamers, they managed to get a decent job and can actually spend some significant cash buying tons of games. In addition, the society attitude about adult playing games are not as strict as in old days. Nowadays, even senator played WoW. Thus the gaming industry targeted adult more and more as the potential buyer.
Conclusion : It's not the gamers becoming dumber, they just become adult.