720 is an old sounding number. It makes it sound like 720p. We all just spent the last 10 years navigating 720p/1080p when shopping for televisions. "Xbox 720" sounds like a product from 2006, expect this next console to not have any number in the title, is what I would guess. Even calling it Xbox 1080 would be shooting itself in the foot, given that we are now in the 4k era.