We can rather tidily solve this by saying orthography and writing systems are artificial methods of representing the spoken language which have prescriptive rules. English spelling certainly shows that it's not difficult to retain many spellings that no longer accurately reflect the pronunciation of the word, if they ever did in the first place. On the other hand, preventing grammatical changes or semantic shift in words over time is impossible. Nobody's ever managed that (perhaps we could find some exceptions among languages that are used in religious or ceremonial contexts primarily and not as someone's regular means of communication).