It is impractical to close wiki topics because it’s difficult to give feedback and suggestions to the topics via replies.
A good approach the Discourse Team has come with for wiki topics is to configure the category to:
a) never close the topic
b) delete replies after 90 days or so
As a result, people can comment on wiki topics, ask questions, give suggestions, and all that will be gradually incorporated into wiki pages to make them better and more comprehensive.
In the same time, as older comments became handled and irrelevant, they will be deleted automatically.
I’ve also been managing an online community with up to 1M views per month, it has a few wiki-topics categories, and the approach described above proved to work very well.
Generally speaking, you should seek for more interaction and feedback from community, not for less. So while Forum is exactly that place for interaction, it doesn’t make much sense to close topics that naturally trigger discussion and questions.
It’s a good idea to close bug reports once fixed, or Q/A topics once a definitive answer is given.
Hope this helps.