Here is a thing I think you are forgetting, how many series do you actually read all the way to the end anyway? There are a lot of series I have started that I stopped not becasue I ran out of books but because it stopped landing. Are these series bad? No. It's just that readers tend to wonder off. A lot of series lose 40% of sales book to book. So if there is a series you do want to read you should buy it. After all, a lot more series end from lack of sales than author inaction.