The presence of nucleus in any somatic, reproductive or floating (corpuscles of blood) cells of an organism is a normal phenomenon. If you see the evolutionary trend among the vertebrates respectively in fishes, amphibians, reptiles, aves, and mammals then the size of RBC decreases but the number increases. So, the nucleus along with mitochondria present in the vertebrates except mammals help them in the maintenance of aerobic respiration thus to leading towards the maximum life-span however in mammals nucleus and mitochondria absent help them in carrying the anaerobic respiration thus leading towards maximum life-span.
It is a natural balance in respiratory function. Presence of neuclated RBC helps in erobic respiration in fish while absence in man helps in anaerobic respiration all targeted at ensuring life span improvement.