I wonder I don't really like the way YouTube implemented it but would using a modified sub management system be useful here on steemit? I think YouTube has it set at something like account activity related to channel over the past year? I'm sure with the info that could be collected from the blockchain (votes, resteems, comments, comment upvotes, comment replies) in relation to X account that is being followed perhaps they could come up with a rule to unsub someone after saying X posts to Y engagements in a ratio, and when the ratio gets outside of a set value they unsub. And with steemits "lifecycle" it's not time but engagement with subscription based. I'm no dev but I do like thinking through problems so I have no idea if this is fesiable.