It's simple: if Sinema kills the Build Back Better bill AND any and all voting rights measures, she will guarantee that her party will lose the Senate this year. If she costs her party the Senate this year, she will have no right to ask members of her party to give her her party's nomination for the Senate in 2024, and she will be totally unable to make any valid argument as to why she'd be of any use in the Senate.

As a member of the minority who'd spend most of her time voting with a GOP majority and getting nothing for her state in exchange for doing so- minotiry senators who vote with the majority are never rewarded by that majority in any way at all- she would be completely powerless and irrelevant.

And it goes w/o saying that she'd never be rewarded by a future GOP majority for defecting to them.