Of course, all this is because everything has been done. Read Chaucer's "The Canterbury Tales". Read one story half way, and ask yourself how it will end. You'll be right. In 1483, there was so little published entertainment that plot twists were unnecessary.
I think what happened is that writing and even publishing has become democratised, in a sense. In the sense that there are many more people alive today, than in Chaucher's time, that can write, and that want to write (because they love reading, or even writing) and that can do it reasonably well, and get their writings published. Inevitably, the majority of those people will not come up with the most original stories, or the best writing, inevitably the majority of what is written will not be the best of what is written. Duh.
So it's not like there's fewer stories to tell now, it's just there's more people writing and 90% of everything is dross, as ever since the dawn of time.
Seen another way, in Chaucer's time you had to be Chaucer to write, and publish. Nowadays, anyone can write and publish. Inevitably, quality degrades, and originality degrades.