Oh yes! To Grandfather or not to Grandfather! That dilemma!
I understand they are still using the same functionality they had at signup time, and are still on the same API version, so not taking advantage of new direct functionality.
Yes, of course there is a bunch of indirect functionality or magic behind the scenes, but from a company lifetime perspective, when you enter into an agreement with a service provider, you would expect a relatively constant delivery of services, at the agreed prices, especially if your needs have not changed.
There are many companies that upon adding new functionality or services, decide to grandfather. New direct functionality comes at new prices, even if you are inherently benefitting from the new stuff.
Three I can think of, Zendesk, Customer.io and Geckoboard grandfathered our plans.
When it is easy to switch providers, grandfathering is not that important, but if you're tightly integrated, which pretty much everyone doing volume on stripe is, then you're screwed. You can't leave.
Even if you negotiate custom pricing, there is a knock on the door one day saying, "new pricing in place. Take it or leave"