You could start by reading the Treaty of Accession from 2003.
At the time of the treaty, joining the monetary union was probably perceived as more of a privilege than an obligation. As such, the convergence criteria are quite stringent. If Poland enacted policy constantly which meant it systematically avoided meeting the convergence criteria, then surely the could indefinitely delay joining, by saying every couple of years, as they have been, "we need more time". Such economic policies aren't necessarily that bad, as the convergence criteria are pretty conservative.
I think Germany has more pressing fights than to demand Poland joins the Euro. And if they really want to press the point, by the time that happens Poland will be in a reasonably strong position to renegotiate: the existing opt-outs make it feasible for them to renegotiate, as well as the fact it may be mutually beneficial for Germany and Poland for the zloty to remain.