There's only two situations I can think of where the Xbox Series X is the last.
1. It under-performs a lot. We're talking less than the Xbox One, and maybe as low as the original Xbox.
2. It still does well, or even really well. But Microsoft really believes the future is in the cloud, and announces Xbox hardware is done. The Xbox brand and studios will survive, but the games will be available on Windows, and maybe some will roll over to Sony and Nintendo in the tenth generation.
Phil Spencer has said that the plan is not for the Xbox Series X to be the last. But of course why would he's say it's the last? Even entertaining the idea that it is the last Xbox, it would be terrible marketing to admit as such before it even releases.
Lifetime Sales Predictions
Switch: 144 million (was 73, then 96, then 113 million, then 125 million)
PS5: 105 million Xbox Series S/X: 60 million
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima