This is a common fantasy among technologists (and a common fear of teachers) when confronted with educational technology. But it won't happen in our lifetimes. It will not be possible to replace teachers with technology until we discover true artificial intelligence.
Educational technology, correctly applied, can be a force multiplier for teachers. So it may mean we can teach more students with fewer teachers. But it also means that access to education will increase (which may cause demand for education to increase).
One way or another, someone will always need to be available to answer students' questions. Books, videos, and clever programs running in web browsers simply can't do that.
Can't we use a model similar to Stack Overflow for answering students' questions? Most questions are bound to repeat, so I don't think there would be so many questions for the experts to answer after a while.