From a lore standpoint, Default MF never became Captain MF. Captain MF was captain of Syren, the ship she used to blow up GP's ship. Then Bilgewater goes into turmoil and she no longer stays Captain. She goes back to bounty hunting, hunting down GP's men alone which is her Default Skin.
At the Harrowing, her old first shipmate even stops calling her Captain as he did when GP was alive.
From the Burning Tides: “Is everything in place, Rafen?” she asked. After all these years, he was still taken aback at how he could never surprise her.“Yes, Captain,” he said."
From Harrowing: “Look, I get it, Sarah. We won Bilgewater and you need to fight to hold onto it, to show everyone you’re better than Gangplank. Well, you’ve done that..."