Summarizing the Third Century Crisis

[Robert Vermaat]

I believe that the concentration of troops at central locations was more due to the fact that the emperors needed them to fight their fellow Romans and also needed to stay in personal contact with them to survive than to any changes in defensive strategy. With respect to the "increase" in the number of cavalry, evidence is shaky and it has been pointed out that the strategic speed of cavalry armies is far lower than that of infantry armies. Again, this may be more of an issue of concentrating "high value" troops closely around the emperor.

Agreed, but the example of the german raiders who broke through the Danube defences and entered Italy unopposed also had a part in that changing strategy. I believe that the development of (stone) walls for many cities and towns was part of that changing view.
About gallienuis, he did not so much increase the number of cavalry, than that he created cavalry units by removing the legionary cavalry and created new cavalry units from them. He is credited with creating the first proto-field army, a model on which later emperors continued.