Campaigns can only be archived at Special:CentralNotice. While the most recent inactive may be worth archiving, we should be able to delete them, for example, for years-old campaigns no longer active that serve no historical purpose. Actually Special:CentralNotice archive is clogged with old campaigns. Thanks.
Archiving them is fine, I think. (In fact, even when you delete a wiki page as well, you are just archiving it; not really deleting it). If you uncheck the "Show archived campaigns" in that page, the archived campaigns would be hidden. Deleting campaigns would not be good for historical purposes.