I think that providing an exit status that is not 0 when zcat is used with an uncompressed file is useful. Though my opinion is less strong regarding whether it should write more text after an error occurred, it’s probably more useful for a process to terminate quickly when an error occurred rather than risk a second error occurring and making troubleshooting harder.

I think that trying to change any existing documented features of widely used utilities will lead to us having less useful software in the future (our time is probably better spent making new programs and new documentation): https://www.jwz.org/doc/worse-is-better.html https://en.wikipedia.org/wiki/Worse_is_better