It’s not very inspiring or heartwarming, but you can make a fairly strong realpolitik case in favor of selling out the Kurds in favor of the Turks. We have few direct national interests in Syria; after all, we lived with the Assads without complaint for decades. Leaving a handful of troops as a tripwire was not sustainable in the long run, and would not have given us meaningful leverage in any future Syrian political settlement. Turkey is an extremely important NATO ally and a natural enemy of Russia. We’ve sold out the Kurds before; they had to know it was coming at some point. And so on.
But you would never do it like this! You would exact the highest possible price from Erdogan in exchange for withdrawing. You would get some security guarantees for the Kurds before you made the deal. You would make sure the withdrawal was accomplished in a safe and systematic way. You would insist that the Turks stop playing footsie with the Russians. You might even demand that Erdogan restore some measure of real democracy in Turkey. And once the deal was done, you would stick with it and avoid offending the Turks. In the event, none of that has happened.
Trump has succeeded in making us look ridiculous and completely unreliable. The big winner, as usual, is Putin; he may not have many resources at his disposal, but he keeps his promises to his friends.