Your comments about Triarii were the first time I've heard it proposed that mobility within the legion stagnated during the 2nd Punic War due to the veteran status of the legionnaires.
Well there's two things going on there. First, the Triarii were just beginning to be phased out during the 3rd century. Hannibal's invasion only accelerated the process, as they were ill suited to fight against Carthage's armies. If you look at Scipio's campaigns, especially in Spain, he uses Triarii in very non-traditional ways. In fact, not far off from how most BGFW players have been using them: as battlesquads. (I'm vague here because I'm drawing on memories that are 10-12 years old at this point; but you can probably research it yourself if you wanted)
Second, the Second Punic War
absolutely would have discombobulated Roman society. Look at the economic upheaval of the US economy in WWII (where we became a soft command economy), and we didn't even fight that war on our territory! Look at what WWII did to Germany, Russia, France, and the rest of the continent. Look what WWI did to them before that.
After Trasimene, Fabian Maximus had Rome build a WWI siege line across the middle of Italy. That trench line is still there today! They drafted every able bodied man they possibly and sent them off to war. It had a huge dislocating effect. And like all major wars, they started talking about armies the way they had talked about divisions (using modern terms). Individual promotions and things like become much less important than bottling up Hannibal.
I am, however, surprised by your ascertation that Hannibal precipitated the process leading to the Marius Reforms - the war finished 100 years before the reforms so I find that one difficult to accept.
Ask any Classics historian who destroyed the Roman Republic and they'll say the same thing: Hannibal. See, the traditional Roman system was to draft landed males. To be eligible to be drafted (and thus be a full fledged citizen), you had to own a certain amount of wealth in land. You had to provide your own arms and armor for when you were drafted.
This system works well for short wars, for the obvious reason that farms don't plow, plant, and harvest themselves. The Second Punic War was sixteen years. You had guys away for 8, 10, 12 years at a time. They return home to a land overgrown and uncultivated, their tools stolen or rusted/rotted, and perhaps even their house fallen in. Plus they had little to no money to buy things like seed and livestock (if there were any available, remember Rome had been on a war footing for a generation).
Meanwhile along comes a nearby patrician, who had been an officer during the war and thus had a higher share of pillage in addition to ancestral holdings that hadn't fallen into disrepair, and he offers to buy the land for pennies on the dollar. But still, pennies on the dollar is more than the soldier could expect to see out of his farm for the next 10-15 years. So he sells and moves into the city.
Now magnify this across Rome. Suddenly, the vast demographic base that Rome had been able to draft withers away. The cities swell, sure, but that population isn't allowed to be drafted.
At the same time, out of the Second Punic War comes the precedent of "Authority with Office." See Scipio Africanus petitioned the Senate during the war to give him an army and he'd invade Spain to cut off Hannibal's supply of manpower. The problem was only Consuls could lead an army ("Authority") but Scipio was too young to be a Consul. So the Senate gave him the authority to lead the army without being consul. However, they demanded that he would pay for it, something he willingly did.
In other words, Scipio was allowed to create a private army with the blessing of the Senate but without any of the limitations of elected office (like having an end date for your term, even dictators had to step down after 6 months). That is a Bad Thing. Armies are always loyal to the person cutting your check. And while wrapping your leader in the cloak of nationalism seems bad to some, wrapping it in hero worship of the general is always worse. Nationalism usually embraces the status quo. Cult of Personality usually boils down to the guy on top promising the masses to "get them what they deserve from them that took it from you."
You see these two trends become exacerbated in the century after the 2nd Punic War, and Marius's Reforms are the logical next step. First and foremost, he allowed landless citizens to be drafted. We may think this not a bad thing today, but the mentality of the time is that people without land (i.e. wealth) are more prone to topple the government and take the land (wealth) by force from those who have it. A solid point when you think about it.
One of the other things the Marian Reforms was an attempt to halt the loyalty to the general of armies. Every soldier who served was given a land grant on retirement. The idea is that people would be loyal to Rome if Rome gave them wealth from the people they conquered.
One flaw: you had to go conquer the land. And conquering armies tend to worship their esteemed leader. The culture of "Authority without Office" had simply taken root too deep. And from that you see the chaos of Sulla and Marius, the 1st Triumvirate, and the 2nd Triumvirate.
Hannibal killed Rome. He buried a blade in her side that took her a hundred years to bleed out, but he killed her.
