First, we need to ask why the EP wants the Polish PM to withdraw the case. If you ask me if you are right, then the law is on your side. In other words, the EP knows that the Polish court, especially, Constitutional Tribunal, will rule that the country's constitution takes precedence over European Union treaties. So they know that this verdict might expose the European Parliament and European Commission as the ones breaking the "rule of law" they bring upon us daily, sometimes twice a day. That, in turn, could call into question the bloc's legal order. Other countries may take a second look at the treaties and discover that, sure, they didn't give the EU all the rights they now usurp.
That is why they want the PM to withdraw the case.