Bo was entering his age 23 season in 2021 when Semien was here. We only had Semien for 1 season and had no other viable options who you could project to replace Bo at SS long term.
At 23 - there's still potential that he improves defensively. Semien himself was pretty awful defensively at a younger age, but improved with experience.
And of course the relationship with the player has an impact - he was a franchise cornerstone guy.
Could they have moved Bo to 2nd last year and put Gimenez at SS? Sure, but prior to last year, I'm not sure it made a lot of sense unless you added someone like Lindor. Who know what conversations the team had with Bo, or how he felt about it. It's all super grey.