Well, the Orks and nids are both very powerful and capable of doing massive hordes, or elitemonstrocities (lootas tear monstrous creatures apart, gotta love str 7 1-3 shot guns plowing into a hive tyrant stupid enough to not bring guards (sadly hitting on 5's so out of 45 shots (max) 15-20 may hit, on 3's to wound another 10 or so. taking his saves he may fail a few. 30 boyz with shootas also works. 60 shots, 20-30 will hit, wounding on 6's, so 10 max may wound, do both in one turn and youve just destroyed a 200+pt model WOO
Though the nids have tonnes of shots that deny ork armor regardless, so shooty nid armies eat orks for breakest.
Since orks reproduce so quickly, and the nids can't hit everywhere. their fight would be a long and hard one. Eventually the nids might lose to much biomass and pull back, or the nids find a way to use the orks regenerative and reproductive abilities to create a limitless source of biomass (send out a spore, it becomes millions of orks, eat all of them but 1, use it to make another million orks... rinse and repeat. thus the orks would be absorbed by a giant ball of bio mass and become a new strain of genestealers. and that would be the end of that.
But point arises, The space marines(this includes ALL VARIANTS) are the main reason the orks are unable to grow at the rate they were designed for, being killed and purged by the billions daily. If this was to stop ubruptly. the orks would grow astonishingly fast and would overwhelm the other races (eldar would be consumed by green, as will tau and imperial gaurd. the necrons would put up a harder fight because of their own staying power but eventually the numbers of greenskins would become just too much.
Now the tyranids have to fight the largest WAAAGH! in the galaxy, A Massive wave of nothing but green, countless ork ships would decend onto the hives and the orks would plow their way through, cutting off the tendrils and stuffing bombsquigs into nornqueens for fun. the docs would learn tyranid weaknesses through examining the very workings of their creation, even engineer new things to experiment "bio-doks" would use tyranid technology to create orks capable of infiltrating hives unoticed and wreak havoc from within, with the hives distracted the orks would hit them like a wall of green. and when the smoke clears, a warcry of every greenskin in the galaxy will be heard, the chaos gods will quiver with fear, as they know that now the tyranids are gone, the eye of terror is next!