It has begun; the decline of retro game collecting.
As someone who got into retro game collecting a few years before it got white hot, it's something I've been following for a while now and retro game collecting is starting to follow the same price/demand trend that nearly every retro collecting fad has experienced over the last 30-years. What's funny is this topic seems to come up nearly every year and while I probably sound like a broken record each time it does, I'll say it again. Retro video game collecting is no different than vintage toy collecting, baseball card collecting, comic book collecting, and Gen 1 and 2 retro game collecting; all of these collecting crazes rose in popularity, prices got out of hand, people's priorities and income changed over time, and the collecting trend died off, typically within 10-years. I knew retro game collecting would be no different and the prices are starting to reflect that.
I once thought of the collecting market as a bubble, but bubbles pop and prices come tumbling down fast. This is more of a slow death as many collectors are starting to get board, take on financial responsibilities (kids, marriage, mortgages. new car, ect), or just get burnt out of the high prices that have lasted for a while now. The overall price trends on Price Charting reflect this happening, and you can see it on ebay as you pointed out. On a personal level, the independent game stores and second hand stores that sell retro games have been getting more of it in, have been holding onto it a lot longer, and in general fewer people seem to be interested in paying ebay prices for retro games. I've also know a lot of collectors and a few large resellers in my area that have liquidated their collections or inventory respectively, and I'm seeing this continue to happen. At least in my area, a lot of retro game stores have also gone out of business, which is partially due to the diminished demand for retro games that was their golden goose just a few years ago. You can almost feel the enthusiasm leaving the hobby as well, and I am completely okay with that.
I've never cared about the value of my games, and to be honest I want games to get as cheap as they can again so I can go out and find a ton that I'd love to own or own again. It should be interesting to see how low prices go when the hobby finally levels out which it is still dropping more and more, and probably will for the next 5 to 10-years before that bottom is reached.