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VGC will revise down their estimations and "Under 35.5M" is the only real option here.

Remember what happened with XB1. It's not sure they will revise it down.

Microsoft didn't want to sell as little as possible back then.

Depends on how long it takes for them to replace the XBS, but sales have completely collapsed and at best they might sell another 3M. In the U.S., which was always Xbox's strongest market by far, sales in 2025 were the lowest for any year since the brand debuted. If Welfare's estimates are on the mark, the XBS sold slightly less in 2025 than the OXbox did in just two months in 2001.

We have an estimated 1.4 million Xbox Series sold in 2025 in the US. Worse than its launch year, which was just November and December 2020. That is also worse  than any year for the Xbox One in the US ignoring after it was replaced and discontinued. Only time an Xbox console in our records sold worse in the US (ignoring after it was replaced) was the Xbox 360's launch year. It is the same story worldwide. 

As far as lifetime figures I agree. I don't see it selling more than a few million more units depending on when Microsoft stops production. 

Something I wonder is how quickly support for the Xbox Series will stop due to just how low its install base is. It wouldn't surprise me if it actually starts declining this year due to its extremely low sales not making up for the amount of people that decide to stop using it. With the small install base and annoyance of having to deal with the Series S I fully expect games will stop coming to it faster than the Xbox One.

If you are taking strictly about 3rd party then for sure.  However,  their biggest money maker is Game Pass and the vast majority of the subs come from the console.  So, they are going to need to keep content going for that system.  Microsoft will also want third parties in GamePass to also support Series consoles.

That is very possible though I hope Microsoft doesn't force their studios to support the console for an overly long period of time. The issue is with them giving up on selling hardware in large numbers they'll need to make up for the Xbox Series user base dwindling over the next few years anyway.

https://ir.take2games.com/static-files/98e1567f-07f8-4e07-9444-2cd954cabe57

~138 M gen 9 consoles. PS5 is 92.2 M Switch 2 is 17.3 M. Series is def under 30 M.