I also share your fear about this imbalance between the one time payment and the need to permanently store the data.
I created the following topic about it: Sacrificial data vs non-permanent data
Basically I proposed that in the event that the network runs out of storage space first the sacrificial chunks are deleted and if it is not enough then the older chunks would be deleted. People could pay some safecoins to update the time stamp of their old files to insure they won’t be deleted. These payments would provide another source of recycled safecoins.
It was replied that this proposal was breaking the network and that the technological progress allows a storage capacity growth large enough to prevent this problem from happening. But I am still unconvinced that an exponential growth of the network will hold forever (IMO this kind of a growth is needed for the new data added each year to finance the storage of past years data).