They explained in the show that the Island is the wellspring of all life/souls on Earth, so destroying the Island would destroy all life on Earth. There is only one protector given immortality, and one guard dog given immortality, they can't just make everyone immortal.
Someone had to manually punch the Number into the Swan because the anomaly disturbed the electrical equipment and automation couldn't be trusted.
I will admit one odd plot hole in the series is the fact that the Others let Desmond stay down in the Swan alone pushing the button, knowing (?) that the fate of the world hung in the balance if he didn't. What if he killed himself? What if he overslept? What if he slipped in the shower and broke his neck?
My fan-wank to this is that since they were watching him on the video the entire time, they were ready to rush into the station the moment they saw Des go down.