Of course, in principle nothing is set in stone for eternity. The objective for monetary policy could also be changed. A democratic procedure for such a change exists, namely an amendment of the Treaty. This is a constitutional process similar to that which originally set the mandate which requires approval by the legislatures of all Member States.
The status of the primary objective of price stability is therefore fundamentally different from that of an inflation target in a direct inflation targeting regime. Inflation targets are typically set by the government of the day for a fixed and relatively short horizon.