What did the first crewed dive to Challenger Deep achieve?

Very little, and that is the point of the story. Jacques Piccard and Don Walsh reached the bottom in the bathyscaphe Trieste in January 1960, at nearly 11,000 metres, and their descent stirred up so much silt that they could see almost nothing. One of the outer window panes had cracked on the way down. They stayed about twenty minutes and came back up.

They did report one thing that mattered enormously. Piccard said he saw a flatfish on the bottom, which was taken as evidence that vertebrate life exists at the greatest depth and pressure on Earth - it was influential in the argument against dumping radioactive waste in ocean trenches on the assumption they were dead zones. Later specialists have doubted the identification and think it was more likely a sea cucumber, since no fish has been confirmed below about 8,200 metres.

Nobody went back for 52 years. James Cameron made the second crewed dive alone in 2012, and Victor Vescovo began repeated dives in 2019 in a submersible designed to do it routinely rather than once. Vescovo found a plastic bag. More people have walked on the Moon than have been to the bottom of the Mariana Trench.