It's not about delayed gratification - it's about predictability.

I know what I'm doing tomorrow if I can afford to wait for a delivery. What will I be doing on a random day next week? next month? Less so!

I order lots of stuff I need on Amazon. Not want, need, and while it’s not life or death usually I need it sooner rather than later. If I can wait, usually I’d just order it elsewhere (including from somewhere like Taobao or Alibaba/Aliexpress and waiting months for it).

And that’s exactly where Amazon is super difficult to replace. The current competition is not other online retailers but coordinating with my wife such that she can look after the kid so I can go sit in traffic for an hour to get to the store and grab a couple of things we need for some quick home repairs, etc.

I've also found that if I am looking at a longer delivery time then I put more thought into evaluating if I really need the product, or if I could make do with something else, or if there is a better product available elsewhere. I then tend to focus on quality and other fitness for purpose instead of which item is on Prime delivery.

Well from a business stand point, I know we have shifted alot of our daily supply things for IT from a "Keep 10 of these instock at all times" to "Just order it from Amazon as needed" due to the faster delivery time

Just as Manufactures have shifted to JIT for many parts on the production line, other business processes have as well.