Look at other parliamentarian governments through out history. Germany under the Wiemar Regime, Japan prior to WW2, Franco’s Spain, Fascist Italy and numerous Central and South American governments.
Quite. That suggests that failure lies in specific local causes, rather than being inevitable with the structure. There are plenty of other successful parliaments which have never descended to dictatorship - Israel, for instance, or most of the ‘Old Commonwealth’ nations where legislatures are based on the Westminster model (Australia, New Zealand etc). Just as there are successful and unsuccessful republics.