This comes up often in Hacker News discussions, though usually without the qualifier I've quoted above, which I appreciate you adding. It's really easy to underestimate how much there is beneath the surface of almost any product.
People start by looking at Khan Academy and thinking "it's just a collection of videos, and even those are hosted on YouTube." But we've also got exercises and articles. Mostly created in-house, in a CMS that can handle the needs of creating math and science content. Translated into dozens of languages mostly by volunteers. Organized into courses based on, in some cases, regional curriculum.
We need to keep track of which content a learner has worked with and which skills they've mastered so that we can help them keep improving.
We connect with The College Board to help with specific SAT prep based on test results you've had.
Accounts can be logged into using Clever, which is common in school districts. They can be connected to Google Classroom, and teachers can assign content (and manage classes on the site).
We offer higher-level reporting for school districts, plus special tooling to get all of the district's students loaded into Khan Academy.
Around all of these features are the inevitable tricky edges. All of this needs to be able to handle millions of users every month.
In the process of doing this port, we _are_ actually deprecating some old and outdated stuff. Our site is 10+ years old. But most of what we have are features used by lots of people all the time.