Sen Sinema's switch to being an independent is viewed as a way to avoid a Democratic primary which she would likely lose. So if she runs as an independent in 2024 it may be a 3-way contest, possibly splitting the Democratic and independent vote and permitting the Republican to win. There's lots of possibilities--if it appears Sinema will run, does that mean the Democratic and Republican primaries will be more favorable to the more radical candidates?
I've also seen discussion over the adverse Senate map for 2024--Tester, Manchin, Brown, Rosen, and AZ are all chancy or adverse. A Democratic who has rose-colored glasses might predict that by Nov. 2024 the economy will have picked up, having dodged a recession next year, and we can compare the country to 1984. And Biden will have become popular, and the Republicans will have been flailing in Congress to do anything. And if the Republican presidential candidate is Trump, or someone equally as unpopular and incompetent (hard to imagine, but don't underestimate Republicans), we could be looking at a landslide.
I estimate the probability of that outcome as <1 percent.