That's ignoring two big points in the OP: that CSV variants are ill-defined and conflicting; and that existing CSV data is too poorly-specified.
There is no reason to try to be "backwards-compatible" with existing CSV files - we don't have a single definition of correctness to use to check that the compatibility is correct. Every attempt to be parse existing data would result in unexpected results or even data loss for some CSVs in the wild, because there is no way to reconcile all the different expectations and specifications that people have for their own CSV data.
There is no reason to try to be "backwards-compatible" with existing CSV files - we don't have a single definition of correctness to use to check that the compatibility is correct. Every attempt to be parse existing data would result in unexpected results or even data loss for some CSVs in the wild, because there is no way to reconcile all the different expectations and specifications that people have for their own CSV data.