Part of trade value is how much time is left on the contract. Why would you give up assets for someone you can sign outright in 7 months, especially if you’re a dumpster fire like the Knicks, or super young and spinning your wheels like the Hawks?
DDR will be the best UFA if he opts out next summer, because AD isn’t going anywhere. There aren’t even any other 20 point scorers available. He’ll get offers in the range that he wants, because it’s a weak FA class, and guys get overpaid in those years.