First of all, github is not the web, and much of the web does not have an API (and I don't have a clue how good the github API is).

But more importantly, even if they have an API: Is there any legally binding committment on their part that that API will be a reliable basis for me to build on for the next 20 years or so, assuming they still exist then? Won't they change it as it suits their needs, in particular their business needs?

A "programming interface" that I can't rely on in the long run is still kindof a user interface, as I have to bother with it whenever it changes. It's a real programming interface only if I can build an abstraction on top of it and essentially forget the layers beneath.