Nobody team-builds in a vacuum so you’re actually right on the money. We drafted Tre and traded for Gaines which made Darby pretty expendable.
Trading Dareus left a hole at DT. Star’s contract was going to be up and McD and co knew it. They didn’t trade Dareus, and then 12 weeks later go, “hey maybe we should sign a DT because we don’t have many, who is available?” That doesn’t mean their ONLY plan at DT was Star, but that’s the one they executed to replace Dareus.