To me, I feel like it's always been more accurate to say that there are "casual" gamers, it's not so much about whether or not a game is casual. There are titles that are being marketed to try and find this elusive market, of course.
(This often feels like a case where the fanbase has decided this terminology exists/has always existed, and now is struggling to define a word that doesn't necessarily apply - which strikes me as some real bottom-up design. Usually you coin a phrase to define a situation or explain a trend you've noticed. This feels more like we've co-opted a word and are now trying to create circumstances where we can use the word in a meaningful way.)
I feel in some circumstances games have been streamlined (I hate the term "dumbed down.") There's certainly been a financial impetus in lowering the barrier to entry for these games. Overall I don't think that's necessarily a terrible thing really. In my view, the best games are simple to grasp yet difficult to master - like Chess. I can tell you how to play the game in ten minutes, but we're going to spend a lifetime mastering it's intricacies. If anything, the graphics arms race has done more to "simplify" games than anything else, I think. Just due to the sheer amount of resources you have to throw into that to stay competitive in that area - and that also generally means smaller levels, less intricate designs, etc.