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You can already achieve this behaviour by pointing the input and output args to the same file. IMHO adding this flag increases complexity of the cli without improving it.

You mean something like "guetzli beens.png" should automaticly create a compressed file called beens.jpg if there is not output file given?

I disagree with @romainmenke on this one. Adding an overwrite flag does improve the program and its CLI, at the cost of only a small increase in complexity. It doesn't break default behaviour since it needs to be explicitly called. It's ideal for situations where a user wants to compress an existing JPG file without ending up with a second copy of the same image.

@gtuk Try guetzli ./my_image.jpeg ./my_image.jpeg. Unless I am mistaken it will replace the original file with the output.

guetzli ./my_image.jpeg ./my_image.jpeg
vs
guetzli --overwrite ./my_image.jpeg

So this adds extra code that has to be maintained without adding functionality.

Such a flag would make sense in a program that does batch compressions and alters filenames to prevent overwrites. Since Guetzli does neither I personally don't really see the added value.