XPY is just like BTC but all the coins have now been mined, so unless there are numerous and large transactions day in and day out, where do the fees come from that generate the rate of return?
This is not correct. All the coins have not been mined. Every POS block is mining new coins. Small amount but they are being mined. The so-called Prime Controllers mine new coins, the so-called Orion Controllers collect the transaction fees for transaction they handle. In the wallet console do a "getblockhash 'block height'" and then use hash and do a "getblock 'hash'" and you will see coins mined every POS block.