Although you did upload about 50 items back in 2012, you’ve not uploaded even half that in any of the subsequent years, your upload numbers have decreased every year since 2012, and you’ve only uploaded 8 items in the past 12 month. From my perspective, your numbers look better than they should! I’m pretty sure there’s nothing wrong with the market, it’s just the unavoidable increase in content which is hard for individual authors to keep up with.
There’s far too many variables for this example to be completely accurate, but if you have 10 items in year X, there are 1000 items on the marketplace and you earn $1000 a month… it stands to reason that if you still have 10 items in year Y, and there are 2000 items on the marketplace… you’re going to make $500 a month. In theory, and with all things being equal… the only way to increase sales (other than improved quality which is a hard one to quantify) is to upload more items this year than you did last year, and more items next year than you did this year, and so on and so forth. If you’re not doing that, and your sales are still climbing or staying the same… then that’s pretty much just down to luck.