You argue very eloquently Je Touche. I won't discuss Leipzig but I have to maintain that Waterloo was THE decisive battle of the Napoleonic wars and it is so simply because it brought them to an end. You are right that Napoleon would PROBABLY have been beaten by the Austrian and Russian armies even if he had won at Waterloo but the fact was that he lost at Waterloo and that was it.
Waterloo had important political ramifications. It strengthened the hand of Britain in Europe and brought peace to Europe (for a while).
I completely agree that few battles are truly decisive but Waterloo was one and Hastings was another. This is not to say that these were the most important drivers of the outcome but that they determined the final outcome. Thus Borodino and Leipzig were more important than Waterloo in deciding the outcome but it was at Waterloo that the Napoleonic wars were ended (and with a bang rather than a whimper) and thus it was the decisive battle.
Another example that springs to mind is Culloden (excuse the parochialism). What killed the Jacobite uprising was the decision to retreat from Derby rather than push onto London and also the failure of the uprising to receive much support in England. But it was Culloden that was decisive because it was here that the Jacobite army was destroyed and that (effectively) ended the Jacobite uprisings and any real hope of the restoration of the Stuart monarchy.
There is also a large number of battles where, had the result gone the other way, history would have been profoundly changed. Bannockburn was one; any of Alexander's battles had he lost them would have changed history: winning Issus wasn't decisive for Alexander but losing it would have been. Had Napoleon won at Waterloo it is unlikely that it would have been decisive but losing Waterloo certainly was.
I rest my case