The war has to end and for that to happen someone has to lose. Someone has to feel defeated. The orcs need to feel defeated. Otherwise being in the Alliance will suck since they're stuck in a never ending cycle of ceasefires and doublespeak with nothing to show for it except more territory and lives lost.
From a detached lore perspective I might agree with you. But Blizzard is ultimately a business, and a business isn't going to have the finale of an expansion involve a massive defeat for half their playerbase. Having so many players experience the largest defeat either faction has ever suffered in WoW is no way of ending an expansion if they want them to return for expansion 5 (and there will be a 5th expansion, Blizzard is already in the planning stages for it).
I've also brought up the point before that we can't consider the Siege of Orgrimmar in a vacuum. So while the Siege itself needs to leave both factions feeling like they won, there's a lot of stuff in the middle that isn't bound by that. In fact I think having the Alliance take the clear upper hand in the mid-to-late expansion might be the perfect way to compel Garrosh to seek outside assistance.
Imagine a situation where the Alliance has recently accomplished a string of impressive and in some cases improbable victories. They ripped half the Horde's offensive into Pandaria to pieces, then sank the Horde fleet bringing reinforcements and are on track to push the Horde completely off the continent within a few months. Cracks are opening up on other fronts as the Alliance pushes further and further into Ashenvale. The Syndicate have turned to the Alliance to protect themselves from the Forsaken, and have turned the fighting in the Northern Eastern Kingdoms firmly into the Alliance's favor. The Horde might be holding their ground on other fronts, but they know that victory is becoming less and less likely and some leaders are even speaking of making peace while the Horde can still negotiate from a relatively strong position.
It is in this dark hour that Garrosh makes his pact with the Mogu to build a new army. One that could not only destroy the Alliance, but make all of Azeroth subservient to the Horde. The other Horde leaders are then forced to make a choice, back Garrosh and become nothing better then the Horde of the First War (demon control or not), or risk everything by rising in rebellion to restore honor to the Horde.