> SFTP came along as basically a working, feature-complete replacement

The FTP standard has a number of features that SFTP doesn't include, such as record-oriented file transfer (STRU R, MODE B), store-unique (STOU – you upload the file, the server picks the name instead of the client). Many of these features are rarely used, and many clients/servers don't even implement them, but SFTP is not a feature-complete replacement for FTP. But there are some environments (e.g. mainframe environments) where these features are used, and these feature gaps make it difficult or impossible to replace FTP with SFTP