At that point you're rapidly approaching the scale where cloud-style on-demand provisioning isn't really helping you (unless you prefer AWS's tooling to any "private cloud", which is legit), and you might as well put a bunch of dedicated servers in colo, right?
That is to say, the reason you can negotiate a discount once you get well past that point (or if you intend to grow well past that point) is because staying on AWS makes increasingly less sense for you.
Depends on the situation - a bunch of dedicated servers means a sizable shift from "operating expenses" like AWS fees to "capital expenses" for hardware. It also means shifting expenses from monthly as-you-go to mostly upfront.
That is to say, the reason you can negotiate a discount once you get well past that point (or if you intend to grow well past that point) is because staying on AWS makes increasingly less sense for you.