The 360 didn't even win with (nearly) everything going in MS's favor. Sure, it dominated the U.S., but it consistently was running behind the PS3 in Europe, and it still flopped in Japan relative to the competition (less than 7% of the total Gen 7 market), this despite numerous Japanese titles being exclusive to the system. Also, the 360 had a one-year head start.
The only way the Xbox 4 could conceivably beat the PS5 is if A) Sony makes some kind of massive blunder (e.g., making the PS5 cost substantially more than the Xbox 4 without making the higher price tag worth it), B) the Xbox 4 has a head start, giving it more time to accumulate sales, and C) Xbox can make serious in-roads in continental Europe.
While the U.S. could easily go either way as it has no particular brand loyalty as a region, I do not foresee any scenario where PlayStation does not enjoy a massive edge over Xbox in Europe, and Xbox is a total nonstarter in Japan. Because of this, the odds of an Xbox system ever being the clear winner in global sales are incredibly low.
Assuming both Sony and MS play their cards right, with no major screw-ups, next gen should be a dead heat in the U.S. between the two, while we should still expect Europe to heavily favor the PS5 and Japan to largely ignore the Xbox 4, leaving the PS5 the clear victor globally, though not quite to the extent the PS4 has been this generation.