I enjoyed this article about DCL sailings out of San Diego. It explains that yes, even departures from SD will be down for the 2020 season it's only because they are trying to prepare other cities for a DCL presence after the three new ships come into play. 2021 will see the port regain the 12 sailing dates that it had lost. According to the article Disney "will be deploying existing liners in new locations to build interest for added itineraries."
Personally, it makes me super happy to hear that SD will be a regular departure port because it will give me an excuse to head back to San Diego. I adore San Diego as a tourist destination, and would be pumped to use my DVC points at the Hotel del Coronado again.