Then we might have an Xbox 5 right now...
No seriously....
One really nice thing about the Xbox 360 was its name. If it had been called the Xbox 2, then the Xbox brand would have appeared to have been one version behind Playstation, which was already on Playstation version 3. It's a silly point, but in marketing terms, it probably mattered. (See the Firefox, Chrome browser version wars since 2010 for example)
Putting a 3 at the end of Xbox and calling it the Xbox 360 cleverly avoided that problem. It was only the second Xbox, but the name made it look like it was competing on equal footing with the PlayStation 3.
If the original Xbox had launched one generation earlier, around the same time as the original Playstation, Microsoft might have followed a much more traditional naming convention: Xbox → Xbox 2 → Xbox 3 → Xbox 4 → Xbox 5
So yeah, in that alternate timeline, we might have an Xbox 2 instead of an Xbox 360, an Xbox 3 instead of an Xbox One, and possibly an Xbox 5 right now.
