Well, in a traditional file system, the basic unit of data is a specific notion of a file which is an untyped blob of octets with limited metadata like size, fi

Well, in a traditional file system, the basic unit of data is a specific notion of a file which is an untyped blob of octets with limited metadata like size, filename, some timestamps, and some very coarse access controls. You could quibble about whether this counts as "replacing the notion of a file", but I could certainly imagine it might be useful to have a system that talks directly to disk whose basic unit of data has much more useful metadata, such as a canonicalized MIME type, much more granular access controls, much more granular access and edit history, etc. Has your mother ever downloaded a file with the wrong extension and been unable to open it? I know I have. Similarly, the abstracted "everything is a file" notion of a file without random access, which includes sockets and named pipes and stuff, is an untyped, unsized stream of octets. Message-oriented protocols like WebSockets and HTTP can and in fact are built on top of that, of course, but it could have been the reverse: instead of a stream of octets, TCP could have been a stream of arbitrarily-large but finitely sized messages. There almost certainly would have been advantages to such an approach, and applications that didn't want the message framing and just wanted a stream of octets could have easily ignored it.