Earlier quoted context omitted.
Pretty much any modern shell (bash, zsh, fish, etc) provides command line completion.
If you really think the completion in current shells is the best there can be then I think you lack imagination. I don't want remembered commands, I want the terminal to tell me what stuff means. I want it to tell me what makes sense in which context. Like the IntelliSense when coding Java or C#. Docs at your fingertips, all relevant options (and none else) available, etc. This is incomparable to the kind of autocomp…
Regarding contextual analysis, the fundamental problem is that command execution relies on a list of strings (argv) with no types or meaning. Each command is responsible for parsing its own arguments, either manually or using a shared library (e.g. getopt). This provides loose coupling and flexibility, but makes introspection really difficult, and features like completions become an afterthought reimplemented in every shell either by special cases for each command or hacks like parsing help messages.
