Seems like Privilege is just not implemented as well as it could be (web app developer here).
As I see it, the idea is that you pay extra to read chapters in advance. Issue is that the pricing scheme makes no sense and deters most readers from even considering it.
My recommendation: Have a scale of some sort where if you want to read a chapter that is X chapters ahead, you pay Y SS. The exact formula for this is up to you.
The simplest strategy? Y = X. So if you read a chapter that is 50 chapters ahead you pay 50 SS on top of the cost of the chapter itself. This works out the same. 2500 / 50 = 50 SS / Chapter, but it's more accessible if you only read a few chapters a head. This let's the reader determine how far they are willing to read ahead. The cheapest price is 500 SS / 15 chapters = ~ 33 SS / chap. This system would allow you to get 33 chapters for that price. If you want closer to your system, it becomes more like Y = 2X, where you pay 2x as many chapters you are ahead by.
There are various other formulas that could be used, for example, Y = ceil(X / 10) * 10, so if you are 1-10 chapters ahead, you pay 10 SS, if you are 11-20 chapters ahead, you pay 20 SS and so on.
I personally spend a few 1000 stones a month, but the cost/benefit just isn't there with how privilege is currently setup), so it's not that I can't afford it, it's just that it's not reasonable. I'd like to be able to read ahead, but I'd want more control over exactly what I'd pay.
This kind of sliding scale approach would let me and others determine exactly how far we are willing to read ahead without being grouped into "5/15/25/50" chapter allotments, where the SS/chap makes little sense (40/33/40/50). I'd rather make that determination on a per chapter basis. If I'm 40 chapters ahead, and it get's boring, I might wait a while until the SS price drops until I feel it's worth resuming, but with this system, losing privilege while way ahead makes for a high risk of dropping the novel and thus no SS for WN. Likewise, if I'm only 10 chapters ahead and I get to a good part, I'll be more inclined to read ahead if I know it's only going to cost me a few SS more per chapter (on average) to read say 10 more chapters, rather than dropping a few 100 SS to read only a bit ahead.
Most chapters are around 5-15 SS, so 33-50 SS means paying for about 2-10 chapters worth of SS, just to read a single chapter ahead, let alone the 5-50 chapters you are actually paying for, in advance, and who knows how many of those are actually worth being ahead on.
Are people going to pay it anyway? Sure, but as a fraction of those who buy SS, it seems like it would be rather low indeed. If it was more a ramp up, I imagine the number of people who would buy them is quite high and even 1 SS extra on a chapter is around 10-20% extra SS per chapter. It's really easy to say "Well I've paid X SS extra to be X chapters a head, meh, I'll just pay (X + 1) SS extra to be X + 1 chapters ahead". And you can bet those that are willing to pay 50 SS per chapter to be 50 chapters ahead are probably going to be 50 chapters ahead or perhaps 100 chapters ahead if the stock pile is that big (in which case 100 chapters is 100 SS extra).
As long as the regular releases draw directly from the stockpile, so authors don't simply only add to the stock pile without releasing anything outside of it, it seems like a solid way to go.
It's a bit shady, but I could see an translator releasing say a 1000 chapter series, with everything initially in stock pile (official releases start the next day/week/month, say 3 chaps a day), and if you binged, you could read the entire 1000 chapters before they are officially released. Would cost just under half a million SS (WN gonna need some higher tiers to buy SS in bulk), but it's possible. If even a single user did that, it would bring in more SS than 100s of others.
This means it would be practical for any sized stockpile, as long as there was a minimum release rate when a novel has a stockpile (1 chapter a day seems like a good absolute min, even when the author isn't expanding the stockpile), then the author is encouraged to keep increasing the stockpile size as the higher it is, the more SS they are likely to get. It also provides a good way to gauge current interest in a novel. A novel where there the top say 10% of readers are 30 chapters ahead is perhaps more interesting to readers than one where the top 10% are only 10 chapters ahead.