There is nothing new under the sun. What people are most likely experiencing is not a lack of quality choices but since they have already experienced so many similar themes - they are finding that what is offered no longer excites them as it once could have.
It don't even need to goes as far as the excitement, the interest by itself is affected by those hundreds of games played.
Passed some time, you aren't anymore looking the game thinking, "yeah, a new incest story", but more, "oh look, again the deeply in love young sister, shy middle sister, and bitchy (but surely secretly in love) older sister, what a surprise". And even if there's a switch in the roles, it's just a switch.
In the end, whatever how interesting can be the story behind, you've to deal with stereotypical characters that kill everything. Either the character will react as expected, and it will be boring, or it will react in a surprising way, but then you'll be pissed because it's also a reaction totally out of its personality.
But is this really surprising in a scene filled mostly by amateurs ? They aren't writers, they don't have the knowledge to goes beyond the stereotypical personalities they found in other games, nor to goes beyond the cliché stories. Be noted that this isn't a criticism against them, just an observation. Or perhaps is it more a fact...
I mean, I'm 51yo, what mean something like 40 years looking at TV shows. And I could say about them what OP said about games, they are more and more shitty. While in fact it's just that they are more predictable now that I have those 40 years of memories as backup. This probably being enforced by the fact that "how to write a good story" books and courses generally rely on the TV shows that were successful in their times. Therefore screenwriters learn how to do their job by mimicking what have been seen by everyone.
And isn't it exactly the same that we can see on the adult gaming scene ? "Hey, this game is successful, I'll study it in order to know how to correctly write my own story", isn't really different from, "this TV show was a worldwide success and is still broadcasted 30 years later, so the next hours will be dedicated to it and what are the keys behind its success".
Then, time to time, someone come with a new idea, and suddenly we feel refreshed, confusing novelty with quality, like we previously confused boredom with lack of quality.