To wrap up this conversation.. thanks for all the input and helping think this through.
For Support topics I remain convinced the goal is to close them within a few weeks in nearly all cases. It’s the only way to be sure that everyone who is asking for help is being answered on a timely basis. Without that, people are going to start losing confidence and will look for answers elsewhere.
I have noticed that if there is a reason to leave a topic open then typically it is also suitable for moving to another category.
But I agree that the idea of automatically closing Support topics 30 days after last reply is not going to work for us. We want to make sure they are actually resolved, and avoid a situation where the category is full of unanswered or only half answered questions.