Earlier quoted context omitted.
No one would design an autocomplete for written language because we remember the words we are going to use, and anyways, if we forget them, there is no hierarchical namespace to browse and navigate. What autocomplete has enabled are deep and broad namespaces in programming languages that do support such a concept. Yes, you are definitely correct that the massive namespaces designed today are only possible because of…
Perhaps not hierarchical namespaces, but there is the knowledge that 'next word must be a verb', 'that verb requires an object', etc.
The right word for that would be code completion or more specific brandings of intellisense, which is completely different from things like T9.