I haven't read anything related to this thread, but I simply wanted to jump in and share a theory I developed about this not that long ago.
I DON'T think that quality is decreasing. In fact, I think it's increasing at an alarming pace. Additionally, due to this increase in quality has caused a sharp increase in both quantity and people who play adult games.
Games that were once the exception such as
The DeLuca Family has become the expectation. While at one point this game was all the rage, it was talked about in general, in other game threads and plastered all over people's signatures. You couldn't avoid this game if you wanted too. Hell, there was even a theory thread at one point! Regardless, despite how popular and loved this game was, and in some ways still is, It's become just another great game in a seemingly endless pot of games filled with similar quality.
(That said, I haven't played this game in forever. So, the quality may have dipped. Either way, you can swap this game with just about any top game like Eternum or My New Family and it'd be the same result).
In addition, games the general consumer claimed were "decent" has increased so severely that we see them so much that we can't really recognize them as decent anymore. Why would we play one of the 500 decent games when there are 50 that are good? As a result, the good would eventually become known as the "decent" and the original decent would become just another part of the "trash" collection.
Then there is also the individual to think about. When I first got into adult games, I'd try just about anything regardless of tag or kink, mostly due to my inexperience and relatively young age (early 20s). Yet, as time went on my tastes began to develop, I learned what I liked and what I didn't. I realized what tags to avoid and which to search. By now, it's basically a science as I have various presets of sorts that I use to discover games.
This has basically created a bubble shield around myself blocking various types of games from my view. This even further changes my viewpoint on what I see and what I deem is "good". Even if you aren't as extreme as I am in filtering, that doesn't necessarily mean that you aren't filtering, because you are. Maybe you judge or search tags, maybe read the summary, maybe the comments, maybe you just play the intro before abandoning the game. Either way, you are filtering games that you may have loved simply because of the filter you placed around yourself.
Lastly, I'll point out that this filter you create isn't bad or negative. It's natural and does help you weed out the undesirables and saves significant amounts of time. But, some tags or fetishes don't always get a lot of love. If you like Male Dom games, your gonna find fewer games than someone looking for Femdom. Or if you search Harem like I do, the amount of quality games is abysmal. However, just because there aren't that many quality harem games, doesn't mean there aren't a lot of quality games in general, but since I search for harem games, it does feel that way at times.