Where would they get valkeries from? I thought they would have moistly their own universes looted tech. As for rebuilding, if the orks base gets overrun with creep, they may not have alot left to rebuild with. And a zerg strategy of base defense isnt totally bonkers, as the zerg main hive area, when done right, can produce armies insanely fast. (4 hatchery/hive structures producing troops at once for example) A few units held back, the solid line of the zerg turrets, and the hatching eggs in the hives themselves means most of the zerg can leave, without leaving the hive even close to defenseless.
Someone mentioned once about the zerg not really having air superiority in the game, even when char is being invaded. The reason for that is simple, because the zerg forces had to be arranged in a beatable way. The maps were setup with solid anti air, OR anti ground defenses, meaning there was always a hole to exploit. Even when they were combined, it was always light enough to muscle through in the end. Watch the cinematic of the start to the invasion of char. The entire battlefleet was nearly wiped out by the zerg anti air. The ground troops were scattered and slaughtered. Only our resident hero raynor managed to keep the entire debacle from turning into a total loss. Without him, its likely every human on char would have been slaughtered.
Again, game mechanics wise, units are produced insanely fast. It takes days to weeks for a new brood to hatch in fluff though, and I think we're digressing to fluff because otherwise Orks can produce entire squads of Boyz, Mega Armored Nobz, and vehicles from similar structures with a little bit of resources, and can literally produce infinite boys at no cost given time to build up.