A long time ago I was promoting the idea of storing date1 of latest payment of each stored chunk (Sacrificial data vs non-permanent data). If a local group runs out of space to store all the copies of a chunk, then the oldest paid chunks in the group would automatically be deleted to make room for the new ones.
A re-payment protocol doesn’t need to be implemented because simply re-uploading a file could do the job. There is no loss of anonymity and anyone caring for a specific piece of data could do it, not just the initial uploader.
All the people in this topic that think the safe network will sustain itself with the natural exponential growth of stored data wouldn’t have to make the such re-payments2.
But people who don’t want to take any risks can periodically reupload files, or maybe do it for specific files.
1: For anybody upset by the notion of date, this could be replaced with section version.
2: In their opinion this safety mechanism will never activate.