Well, I think I've given my opinions on these matters sufficiently, and I do agree someone would be arguing for more vigorous preparation. The thing is, it all flies in the face of centuries of established policy.Trade is irrelevant,the problem is that this is an obvious strategic blunder.Any general who worth their two cents could have pointed out the problems of having another country's borders getting too close to your capital.The fact that they didn't bother to fortify the Alps is stupid.There would have been some input motion by the generals to at least garrison the place.
Even if the Romans don't come in force,they can cause enough trouble if they periodically sent small forces to raid the countryside around Massalia.
Now that would be very strange indeed! Rome is not some unruly coalition of wild tribes after all; it is a highly centralized state run by paranoid strategists. Surely the Romans have spies of some kind in Massalia, and realize that the complacent peace party there is playing into their hands. What possible gain could Rome realize with sporadic forays? They need to lull the fat and lazy League into as much false sense of security as possible the better to catch them by surprise when their trap is ready.
When did Romans ever behave like that? When they send small forces, it is not to raid but to conquer (or intimidate into a client relationship) someone they judge can be beaten easily. Or to punish raids by backward tribes.
Conceivably they are doing just this kind of thing--against the mountaineer tribes, who are not in the League. (But I'd argue, the friends of some inside it). Crossing over the ridge into actual League territory is not something they'd do piecemeal though. When they come, they will come intending to win and win big. I only hope and trust they will come too early--or more fundamentally, with just Italy in hand and most of that quite recently, they don't really have the force it takes to break the League, even with Carthage giving it their all beside them, but they miscalculate they do because they are thinking of mainly just having to conquer the city of Massalia itself, and don't understand that League power has a far broader base due to the unusually consensual nature of its expansion.
Not to exaggerate League innocence too far, they aren't fluffy little lambs. But more raccoon like than wolf like! Half the peoples the League has faced over its history have been enemies and treated severely when beaten. But the other half have been the former enemies of those the League takes down, and have been recruited into League society for a share of spoils. They are the ones whose descendants are still around with their old identity, more or less, and that identity includes membership, hence loyalty. That's my theory anyway! Most of the losers have long been assimilated into the lower levels of winner societies, and by now perhaps some individuals have risen high--only the most recent victims are still hanging around with both desire and possible means of vengeance still hot among them.
If the Romans properly understood the true nature of League society they would recoil, realizing that the task of breaking the League is the task of conquering it in full, and for that they need numbers they just don't have. But they have their own blind spots as the League leadership has theirs, mainly in the matter of assuming other people think basically the same way you without adequately checking that theory.