Maybe Elm itself is a killer app, but certainly not elm-ui. I don't think Datomic is a killer app too. Its certainly not comparable to Rails/OSes at scale
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).
Datomic shards naturally though – Datomic queries are functions that take database values as input. You can pass multiple database values as input and join across them. This is a first-class construct. I don't remember if this works in Cloud but it definitely works in onprem.
