Sales for book 1 are naturally going to wane by the time you get to book 10 and newcomers to the series, a lot of them are going to walk away considering the cost of entry to catch up. By providing discounted early books in the series, it gets more people into it and provides higher sales for the full-price books that you're writing now.
I haven't found that to be the case.
There is always drop off as people move down the series, but each new book brings in new readers. This is where KU comes into it's own though, as the cost of binge reading the entire series so far is a lot less.
The trick with long series is having multiple entry points, and some pointers in every book back to the beginning.
Personally, I'm now in favour of a maximum of 9 books in a series, (after which the drop off intensifies), then start another one which is either a sequel, or a spin off. The latter are good because you can origin a new character, and point back to the first series at the same time. Another 9 books, and you spin off again.
That's the theory though. I'm doing trilogies at the moment for series 6 and 7. And the book I'm writing now has a solid tie back to series 2.