OS X, a few years back, replaced “Save As” with “Duplicate” . This goes along with autosave, and is a good simplification.
To begin with, I don’t always want the duplicate saved . I might keep it as a temporary reference, say to refer to an earlier part of a long document while I edit a latter part. Or as a temporary backup before I start making significant edits I’m not sure will work out well, so that I can roll back later.
Or I may just want to make a temporary edit for printing. Rather than making the change, printing it, waiting for it to finish, going to the printer and picking up the printout, and then undoing the change, it’s simpler to just Duplicate the document, edit the duplicate, print it, and close the window. I may want to leave the copy that is to be printed aside for a while while I do something else, rather interrupting my train of thoughts for the printing.
When I save a file as another file, Save As keep the second file open on screen, closing the first one. But I may want to continue editing the first one.
Some editors work around that by offering yet another menu item, Save Copy As, which is the same as Save As but keep the first file open after the save is done. This is even more complexity  and a layer of band-aid on a broken system. Duplicate does it matter, by opening a new window for the new file.
It’s also confusing to have the same window represent different files at different times. When I undo some changes after saving to a different file, and save, I end up with a situation where undoing an edit originally made to one file affects another file. Save as breaks Undo.
Save As is broken. It’s good that Apple got rid of it .
 You can still Option-click the File menu to get Save As.
 With autosave, changes are saved to a temporary file even if you don’t explicitly save it, so there’s no downside.
 Some editors also offer an Export menu item, which makes no sense — I never where to find a particular file type in Save As or Export, or both.
 Most UX innovations come from Apple.