If a text file has the same number of commas, tabs or semicolons on every line it most probably is (but, obviously, is not guaranteed to be) a CSV/TSV/SSV.
Defining every flavour of these is hardly possible with a simple command line so I would rather let the user to specify an entire configuration file for this. We probably need an entire CSV schema language.
That's already not true for Quoted entries that contain the separator. Which I think is a common CSV use case.
But I do agree you could have a heuristic. E.g. ends in .csv and contains a lot more commas/semicolons/tabs than you would expect in normal text in the first 1-5 lines.
You could still have the flag as a fallback when you need something that's completely reliable.
Defining every flavour of these is hardly possible with a simple command line so I would rather let the user to specify an entire configuration file for this. We probably need an entire CSV schema language.