The angle by which CSV is simple is in the flexibility of input...that is, it's a very decent authoring format, when sent into a system that knows how to import

The angle by which CSV is simple is in the flexibility of input...that is, it's a very decent authoring format, when sent into a system that knows how to import and validate "that kind of CSV" and treat it the way all user interfaces should treat input - with an eye towards telling the user where they screwed up. Once you've got the data into a processing pipeline you do need additional semantic structure to not lose integrity, which I think Parquet is fine for, but any SQL schema will also do.