Good books are a kind of literacy training. While you read, you train your language skills by absorbing the norms of spelling and punctuation. And if such training is abandoned, the state of literacy will deteriorate in the same way as the physical form of a person accustomed to the gym would deteriorate.
I was convinced by my example and the example of many acquaintances, who are increasingly reading various kinds of Internet instead of books.
If a person is truly literate, he "earned his own diploma" by reading, learning poetry by heart, memorizing the rules. For such a person, literacy itself is very simple, for him it seems to follow from what is happening, does not even allow him to think about himself, a person already sees everything in the correct light, with commas separated. A literate person is always learning, always improving, always looking for an opportunity to improve his literacy.
If a person taught to read and write in order to get 3 (satisfactory) in the Russian language in the exam, then the diploma from such a person will quickly fly away to the one who values order and diligence.