Datomic is really neat but, for any system that operates at significant scale, I think the approximately-10-billion-datom capacity is probably too great a concern.

For example: Stop&Shop has 415 stores, and

  365 days * 415 stores * 100 purchases per day * 50 datoms per purchase

will fill up your system in 14 years without even spending datoms on inventory and the like. And that "100 purchases per day" could be low by a factor of 5 or 10 (I don't know).