The argument is only concerned with legislators who do care about democracy and makes no mention of others. Even if only 1% of legislators care about democracy, it could still be important that this 1% don’t propose laws that could hurt it.

The premises already tell us that people tend to believe the assumptions that legislators appear to have, so it is not necessary to take this idea any further. In fact, if people don’t share the beliefs of the prominent and powerful generally and only care about what legislators appear to think, that could strengthen the argument by putting more importance on legislators’ actions.

We don’t care if legislators also seem to value democracy, only that the laws they propose seem to assume people can’t look after themselves. We also only need the legislators’ proposed laws to show this assumption, not their actions in general.

(D) is both too strong and irrelevant. We only care if banning these actions gives the appearance that legislators think people can’t take care of themselves. We don’t care how common or uncommon these actions actually are.

If legislators appear to be guided by the assumption that people can’t look after themselves, then it will likely achieve widespread acceptance. If it achieves widespread acceptance, then democracy will be harmed. This bridges the gap between premises and conclusion, so it is necessary.