After all coins are minted as far as I know the rewards will come from the transactions fees. At the moment we have 4.5 to 6.0% annual return of staked ADAs. When and how this will change after we reach 45 000 000 000 coins in circulation?
To start, all ADA does already exist and is accounted for, as seen in the Introduction section on page 4 of the Cardano ledger spec. The spec states that, from genesis, there was about 13,000,000,000 ADA locked up in reserves (monetary expansion), which are to be slowly released on a per epoch basis at a rate of decay that is defined by a network parameter, rho. With the current setting of 0.003, it should take over 100 years before these reserves are depleted.
How the APY will change after the reserves are depleted depends mainly on the transaction volume because transaction fees are what will pay the rewards after the reserves are depleted. The reserve depletion is so far out in the future that it is difficult to say how the APY will be effected when that happens.
In ADA all coins are already minted. The total supply is 45 billion tokens. About 13 billion coins are not used and are "locked" in the treasury. It is used to be able to pay out the rewards for every block created by the stakepool and from where you will get delegators' awards. Because there not so many transactions to pay for, all the block awards the missing coins are used from the treasury. Once the transaction fees are sufficient to pay out all the block rewards, the treasury will only be used to fund proposals in what is now called Catalyst which in due time will start to solve the Voltaire stage. When Voltaire is ready it will be up to the community what to do with the rest of all ADA from Treasury. And this is a very interesting idea to vote the greedy out in favor of the Cardano Ecosystem to be sustainable.