Is PostgreSQL appropriate for processing this large but simple dataset?
Seems like your x,y is a fixed pair. So you can combine them to form a single key. If all your access is based on a key, then the NoSQL dbs can fit your bill. Some NoSQL dbs offer range queries too but that is not their biggest strength. 

The limitation with the basic/standard RDBMS systems are that they are monolithic. You need a powerful machine to store your 15/40/60 billion objects on a single machine. You will also end up paying cost in terms of performance for the full ACID capabilities of the regular RDBMS systems even though you know that you dont need them. 

NoSQL dbs which are inherently designed keeping distributed systems in mind can help greatly here. When you want to grow from 15->40->60 billion objects, probably you just have throw more machines into the cluster. When the data gets sharded, the amount of data stored on each machine is less (making the index sizes etc smaller). This can give you better read performance.