He wasnt god mode at the end... If you remember his god mode started fading towards the end, by the time he was in space trying to stop the blast from hitting earth he was in regular form. Also as supporting evidence if you recall when they first introduced vegeta and nappa after Raditz dies they were on another planet, as they left the opened their pods in space and destroyed the planet. I think it's a un-said saiyan development thing. At that point goku hadn't been off earth yet like vegeta so he hadnt developed that capability to survive in space. More supporting evidence, when vegeta told everyone of when he finally ascended and became a super saiyan, he was training on a far away planet (similar to the moon) no trees of actual life on the planet aka no oxygen, he has been training there the whole time in space.