Any political argument, hey, just about any argument, is at some level unproveable because:

• There is and can never be a generally accepted view of what is good. 
• Politics is not a science.  In science you construct a comparison between two situations, the one you want to test and another, the control, which is in someway different.  And then you replicate the experiment.  This is not possible in politics.  You can get close.  Communist East Germany and freeish West Germany were close but there were still many differences other than their regimes (size, history, geography, starting position etc).  And replication would be almost impossible.  Even if West Germans were prepared to go back to the Adenauer regime (I assume that East Germans would not want to go back to Ulbricht) I doubt if they would be prepared to go back to the bombed out remnants that they started with. The same is true in a court of law.  That is why we have the phrase “Proved , beyond reasonable doubt.” Ultimately, the reader has to decide for himself what comparisons are reasonable and what aren’t.
• You can never be in possession of all the facts.  This is especially true of things from the past.
• The facts are frequently in dispute.
• Even where the facts are not in dispute they can often be used on both sides of the argument, often, again, depending on what one regards as good.  So, for instance, if, say, attendance at football matches has gone up, whether you think this is a good thing or not depends on whether you think attendance at football matches is a good thing or not.

Nothing yet but you never know...