There is a great deal of material that is not available on the Internet, and will never be available there. Some very important technical material such as engineering standards are available only when you cough up a wad of cash.
You’re right about availability of information. Once a book is purchased, it’s available forever. It may get shabby or dated, but it’s yours. Even if the online materials are comprehensive, access can become an issue. Publishers of school text books generally have an online version available, but it’s limited access. Most publishers provide it as part of a book/e-text package for about 6 years. If a school has only e-text and can’t pay the ongoing fees, the kids end up with nothing.