@feha obviously not, as tough as Nijisuke is Hokuto COULD kill him whenever he wanted, but the collateral damage would be too much so wouldnt be worth it neither of them could outlive the planet, especially Kuroha whos longevity is based around luck
the simple answer is that in a world of gods and magic the earth will still be there in a gazillion years or maybe 100 million years is when the earth will end and thats why her lifespan ends there
also Nijisuke's lifespan isnt infinite becuase he doesnt have one! it was supposed to end before he was even born, he could die at any time and noone has any way to tell but hes so strong that he survives regardless