Downgrade Delta Skymiles Platinum

I am looking to downgrade my card. Is that possible? The reason is that Delta is terribly expensive, and the miles no longer have the value that they used to for me. I live in a Delta hub, and it has become crazy expensive to fly out of this airport with them. However, this is my oldest credit account, and I don't want to lose the history that affects my credit rating.

Can I just move to a non-delta card?