But we only have one more year with Bogaerts, and Mayer is 18.
Mayer had a good 100 PA run down in the complex league, and I think we're all very high on him. But optimistically Mayer becomes a factor in 26-man roster planning sometime around 2024, and the optimistic case for when we would expect him to take the bulk of the SS playing time would be 2025 — and again, that's if things go smoothly. If he moves really quickly (like, say, Mookie Betts did) that has him getting called up in late 2024, but that required the perfect storm of an imploding big league team and Betts hitting .350/.450/.550 in AA. That's not something you plan for.
How likely is it that any of these players — Bogaerts (29), Correa (27), Seager (28), Baez (29), Semien (31), Story (29)... — is still playing SS at a high level by 2025? Suppose we sign Corey Seager to the 8/$240m deal Fangraphs' Ben Clemens thinks he'll get. By 2025, if/when Mayer is ready, it's probably time for Seager to play somewhere else anyway.
In any case, I don't think anyone thinks any of these guys are approaching Betts' 12/$365m deal. Maybe the AAV will be similar, but no one is signing Corey Seager or Trevor Story through their age 40 seasons.