That might be an option. I personally would be fine with that but I've noticed that many / most users get very upset about the notion that posts / communities / users are impermanent ?
Another solution is to simply promote these dead communities - if anyone is interested in warming them up then they should do so. If they're consistent then after a few months ask existing mods to add them as mods, or ask admins to do so if the mods are not responsive.
This approach runs the risk that the person doing the work may not become a mod, but honestly I don't think being a mod should be the objective of creating a community.