For about a year or two, windows had an amazing search from the menu that used a blazing fast index search to search files, directories, and file contents locally and almost instantaneously. It was a glorious thing.
I cannot think of a case in which a user would not need to distinguish between web search and file search (other than the convenience of a single click). I do use a unified search on my phone that includes files, apps, and contacts, and if it's not in any of those, it will launch a web search using the query. That is more than adequate. If it were performing the web search in real time, I wouldn't be able to easily access apps and contacts, and the results would slow and change while typing.