The Senate didn't surrender, and still maintained the loyalty of the wider governing apparatus that collected resources for its campaigns. To be able to raise two dozen legions after the losses suffered in the first three years of the Second Punic War, suggests to me an extraordinary level of social cohesion and stability of government rule.

I believe you are incorrect about the defections as well. One source supporting this notion is Adrian Goldsworthy's "The Fall of Carthage: The Punic Wars 265–146 BC", which, by way of Wikipedia, claims:

https://en.wikipedia.org/wiki/Second_Punic_War#Hannibal_cros...

However, the majority of Rome's allies remained loyal, including many in southern Italy.