I've seen several people ask why a 51D Senate is meaningfully better than a 50D Senate and nobody has given the correct answer: with 51Ds the party can openly primary Sinema without risking control of the Senate.

Risking control how? Do you think she'd switch parties? I don't think she'd have much chance in a Republican primary in Arizona. (Democrats holding the senate in 2024 will be tough either way; it's the most D-leaning of the three classes. See dbaron.org/senate-classes/ .)