Don't know about duty but I think everyone has a right to expect help and a right to give help. It makes for a better world where these rights are regularly exercised.
> everyone has a right to expect help and a right to give help
This might make sense when asking a stranger for directions or for someone to help with your car problem, because the relationship is 1:1.
But this breaks down in contexts where scale is a factor.
When assisting users of your project, the relationship is 1:many. And if the project is popular, it is unfair to expect the author to be helping so many people.
I don't really see how 1:to many makes any difference in principle. Perhaps you were thinking of the amount of effort to help the many? As in 1:1 case it is up to the helper to decide how many resources, if any, to spend on helping. A society where the default position is that people deserve help is different from one that is based on the survival of the fittest.
Even if the default position is to help, it can't be unconditional, there are limits. You don't have a right to expect help when it places an unreasonable burden on the helper.
I upvoted you because I think it's an interesting position, but you should realize that adding duties will discourage people from doing any contributions at all. It starts to feel like a case of "no good deed going unpunished".
The alternative to giving away your personal time for free to an open-source project is gouging people on vital medication? That doesn't seem true in any sense.