That's not necessarily true... You could have the MM world at peace, with each character you helped or didn't help in the timeline reacting accordingly. Also, if you wanted to change the past, you could play the Song of Time like usual. The only problems I can see are that you wouldn't be able to use the Song of Double Time or the Song of Slow Time (or whatever they were called, I can't remember) after you beat Majora.

__________________
"It's easy to cry that you're beaten, and die;
It's easy to crawfish and crawl,
But to fight and to fight, when hope's out of sight-
Why, that's the best game of them all!
And though you come out of each gruelling bout
All broken and beaten and scarred,
Just have one more try- it's dead easy to die!
It's the keeping-on-living that's hard."