No, DL will not be joining *A. They are one of the strong carriers in ST. They are not going to give that up to compete with UA/CO in *A.

That said, have you already purchased your tickets to CPT? If not, maybe you would consider flying with a different alliance (if you are looking only at mileage redemption--if you are looking at elite status, DL is very good within the US, but not sure of your travel patterns).