Let's try this: Which modern secondary source created the term Marian Reforms? It sure as heck wasn't anyone in the ancient and medieval period, its a modern term. Before you state that Delbruck was the first to question it, find out who first wrote about it, and, more importantly, when.Might as well add this, not only for a point of interest but to further highlight how advanced Delbruck was for the time [and not a hack simply lying intentionally to get a certain point across]. The late Emilio Gabba, who was a strong scholarly authority on the Republic [he wrote, for example, the chapter for the Social War in the CAH], attributes Delbruck with first understanding or suggesting that the reforms to the Roman army during the time of Marius were actually part of a longer and gradual change, rather than the sudden revolution or change that was commonly accepted in the day [and given by the ancient sources]:
"As a result, the dilectus of 107, as Delbruck has already tentatively surmised, takes on a meaning somewhat different from that which has been commonly accepted, and seems less of a revolution or an innovation..."
Gabba, Emilio. Republican Rome, the Army and the Allies. Translated by P. J. Cuff (California: University of California Press, 1976), 1.
The quote from Delbruck is provided by Gabba himself, which runs as follows:
"Delbruck had long ago set it in its true form:The essential distinction is whether in the second century the composition of the Roman army was fundamentally restricted to the sons of the middle class, or whether it was already in essence a professional army which only in practice (in so far as the proletarii would have gone into the fleet if there was recruitment for it) retained a certain peasant-proprietor character. In the former case Marius' reform would have set the army on a quite different basis and would have introduced something quite new; in the latter it would have provided only the form to correspond with the reality which had already existed, for whatever remained of the peasant-proprietor character was not completely removed even by Marius but can only have gradually died out.[...] To repeat the words of Delbruck and to sum up, the Marian reform of the levy 'provided the form to correspond with the reality that already existed'."Gabba, Republican Rome, the Army and the Allies, 12.
This is a pretty important innovation that can't be attributed to a man who apparently skimmed through the sources quickly to create an agenda based work, nor does the fact that he personally went out of his way to gain topographical reports of battlefields or even visit them himself really help your argument. Delbruck was extremely influential to multiple areas of military history, the main two probably being the case for lower army numbers and critically evaluating the sources, the other as Keegan himself notes, that Delbruck actually surveyed the battlefield.