mr dakka has the jist of it. if all the orks ever united under a single warboss they would destroy all other races. it written in various books and codexs etc within the IP. the main problem is they love fighting to much and they have command issues. if they dont have other races to fight they will fight each other. also the imperium distract the orks by getting splinters of various tyranid 'fleets' to attack the orks. this can only really end badly for the imperium though as orks get tougher, bigger and stronger by endlessly fighting tougher oppenents and nids get stronger and take on characteristics from things the consume. so you will get either big mean bad ass orks or big mean bad ass nids, either way the imperium is screwed......which is good.
The orks were created as a brainless barbaric race to fight the necrons this is because the people who created them knew they would not be able to stop the beast they have created. They made the orks to strong but because of their need to fight they don't have common goals. This makes it impossible for them to strike together. Unless a legendary warboss grabbed em all and told them to conquer the galaxy. In this case orks would win and then kill each other because of their need to fight.
Maybe the Orks are already the dominant faction, you ever wonder why in the grim dark future of the forty-first millennium there is only war? Everyone fights an always losing stalemate, with no side ever truly conquering the others. The Imperium will never win, Chaos will always be lurking, more Necrons will be found, and so on. The Orks made all the races in 40k so they would always have someone to fight. There will always be someone for the Orks to crush. The Orks already won.
I'm da hand of Gork and Mork, dey sent me to rouse up da boyz to crush and kill ‘cos da boyz forgot what dere ‘ere for. I woz one of da boyz till da godz smashed me in da ‘ead an’ I ‘membered dat Orks is meant to conquer and make slaves of everyfing they don’t kill. I’m da profit of da Waaagh an’ whole worlds burn in my boot prints. On Armour-Geddem, I led da boyz through da fire deserts and smashed da humies’ metal cities to scrap. I fought Yarik, old one-eye at Tarturus, an’ he fought good but we smashed iz city too. I’m death to anyfing dat walks or crawls, where I go nothin’ stands in my way. We crushed da stunties on Golgotha, an’ we caught old one-eye when da speed freeks blew da humies’ big tanks ta bits. I let ‘im go ‘cause good enemies iz ‘ard to find, an Orks need enemies ta fight like they need meat ta eat an’ grog ta drink. I iz more cunnin’ than a grot an’ more killy than a dread, da boyz dat follow me can’t be beat. On Pissenah we jumped da marine-boyz an’ our bosspoles was covered in da helmets we took from da dead ‘uns. We burned dere port an’ killed dere bosses an’ left nothin’ but ruins behind. I’m Warlord Ghazghkull Mag Uruk Thraka an’ I speak wiv da word of da gods. We iz gonna stomp da ‘ooniverse flat an’ kill anyfing that fights back. We iz gonna do this coz’ we’re Orks an’ we was made ta fight an’ win! --Ghazghkull Mag Uruk Thraka The Orks are the pinnacle of creation. For them, the great struggle is won. They have evolved a society which knows no stress or angst. Who are we to judge them? We Eldar who have failed, or the Humans, on the road to ruin in their turn? And why? Because we sought answers to questions that an Ork wouldn't even bother to ask! We see a culture that is strong and despise it as crude. --Uthan the Perverse, a controversial Eldar philosopher
