Great song!
Some of this sounds like it was sound effects from X1.
Now, about the whole "End at X5" thing...It was because Megaman Zero was supposed to start about 100 years after that...also, Zero was originally supposed to be X in the very beginning...but he looked nothing like the classic, so they made him Zero and made the X we see today.
Also, X6, X7, X8 and Command Mission wouldn't exist if they hadn't went without Inafune's permission.