I share your worries of getting the economics wrong @SeanyMc it is an important part of the system, and as you say, hard to change once we’re up and running. I’ll add some thoughts.
Remember that sharing 500GB should give you access to a lot more than 500GB of storage, thanks to deduplication. If 1000 people provide 500GB of storage space each, they will be able to store a lot more than 500GB each because a lot of the data they want to store will be the same.
You could say that the network creates a lot of new, free space. Giving this space away for free makes a lot of sense to me. If every account has to pay a tiny amount when created as suggested here this would give us a model where you get X amount of storage for a fixed amount (practically free - or actually completely free a lot of the time when someone else pays it by sending you an invite) while you might have to start paying when exceeding X.
I personally believe that being able to use the network without providing storage is important and something we should strive for.