-f --force
              If the input data is not in a format recognized by gzip, and if the option --stdout is also given,
              copy the input data without change to the standard output: let zcat behave as cat.

I don’t know why this isn’t the top comment. I guess there might be some scenario where you’d want to know about non-gzip files where you don’t expect them so changing the defaults would probably cause some subtle breakage. For shell use though, just an alias could be used; alias zcat=gzip -cdf