Scholar: The purpose of a law is to deter certain actions by threatening to punish those performing the actions.
Conclusion (Therefore)
A successful legal system prohibits only those few behaviors that citizens find absolute intolerable.
Evidence
The purpose of laws in a legal system is to deter bad actions by threatening punishment if you perform them, but this only works if people think they're likely to get punished for committing those actions, and the more behaviors that are criminalized, the less likely someone will be caught and punished.
Evaluate
When we read Role and Main Conclusion we anticipate the conclusion being "hidden" by appearing as the First Sentence or as some Rebuttal / Pivot moment.
But this first sentence is not supported, so it can't be a conclusion. And the pivot sentence prefaced by "But" is also not supported, so it can't be a conclusion.
Meanwhile, we have a very conspicuous conclusion indicator on the last sentence (we know any time we see so, thus, hence, therefore that it will be a conclusion, even if it's not the main one).
But in this case, it's the main conclusion, because the other three ideas combine to get us there. Why do successful legal systems want to limit the number of criminalized actions to the most intolerable ones?
Because laws only work as a threat if people think they're likely to be caught and punished, and the more actions we criminalize the less likely people are to be caught / punished.
Goal
We're being asked about the Role of the first claim. We'd say it's part of the Support for the conclusion in the final sentence.