https://blog.kozubik.com/john_kozubik/2009/11/flat-rate-stor...
... and the key concept is aligned interests:
"... paying a flat rate for unlimited storage, or transfer, pits you against your provider in an antagonistic relationship. This is not the kind of relationship you want to have with someone providing critical functions."
When you are doing something important you want your interests to be aligned with your provider.
If you pay for every bit you store, your provider will enthusiastically enable you to store as much as you can. Your interests are aligned. If you're paying flat-rate unlimited, the relationship is an antagonistic one and the provider will do all manner of sneaky, underhanded - even outright hostile - things to stop you from storing more bits.