I have been paying attention to Sinema's career for 15 years. I don't think there is another politician currently active that more closely aligns with my political views than Sinema. She started out Green Party, but ran as a democrat because the chance of her winning as Green Party was Zero. Nothing has changed. She has run as a democrat simply because it has been her best chance of winning.
To my knowledge, all Independents who hold office won their first election as a major party and then switched to independent once in office. This really is no different than Bernie Sanders. The fact that so many Democrats seem to be POed of her switching to independent is a glaring spotlight on exactly what is wrong with American politics today. SHE is not changing. Only the box that is checked on her voter registration. She is still much, much more aligned with the Democrats than Republicans. Warnock's win gives her the freedom to do this without pissing of democrat leadership to much.
Democrats are mad at her for not voting to end the filibuster, and for questioning the scope of spending bills. The latter is what *every* senator needs to be doing, regardless of party. We do it for our own household budgets - I am sure few of us go to a car dealer and say to the salesman "whatever you want to sell me is fine with me". It's not like the government has extra revenue to throw around. As for the filibuster - voting to end the filibuster is how we wound up with such partisan SC justices. Had the GOP won back the senate with 51 seats, a vote to end the filibuster could have come back to bite the democrats real hard.