If Starfield reviews well, Xbox sales will increase, no doubt. If the next couple of exclusives review well that'll bring more people in, etc.
If Starfield review wells then it will sell millions... on PC!
Microsoft will need to release consistently good exclusives games on a regular basis (2 or 3 per year) to have any chance of dominating Nintendo or Sony in the videogames market in my opinion. They could do this but they need to commit to making good games rather than telling everyone what great value GamePass is!
They had that momentum in the Xbox 360 era but lost it all with the disastrous unveiling of the underpowered and expensive Xbox One (TV! TV! TV!) vs. the PS4 because of Kinect (look how well that went!) and a slow decline in the number of exclusive games in favour of focusing on their core Gears/Halo/Forza releases and live services. Microsoft could have been a market leader now in the videogames industry if they had actually built on the success they had with the 360, despite that system having a bad start with the red ring of death fiasco.
Just look at Sony and the PS3. It was expensive, heavily criticised for being hard to develop for and yet they still went on to huge success with the PS4 and PS5. Sony had built up a strong reputation with gamers and a large catalogue of quality releases from the PS1 and PS2 which certainly helped the PS3 through the initial rough patch. The Xbox One was Microsoft's PS3 in many ways but instead of making amends, they just became complacent and decided to focus on GamePass and backward compatibility for the Xbox Series consoles instead of quality exclusive game releases.
I say good luck to Microsoft. I would absolutely love them to return to being that company that released the original Xbox, a console I have very fond memories of because it felt like a spiritual successor to the underrated Dreamcast.