When you die in Soul Society, I think you go back to Earth. In a sense, it could be a karmic cycle of reincarnation. Soul Society and Earth balance each other in a time glass. If human souls are destined to go back to soul society, it makes sense if the dead in soul society have to go back to Earth as well. The Shinigami are definitely not immortal. We're not talking Elves here, merely extraordinarily long lived spiritual entities.
Otherwise the Byakuya wouldn't be the only clansmen of the Kuchiki left, he's mum and dad would be alive as well as any other Kuchikis throughout the aeons.
That's why the Quincy's were exterminated. They permanently kill Hollows means the number of souls in the population of Earth and Soul Society decreases until there are no more souls to go between. In a sense its like a gene pool. If you keep killing the species, pretty soon they will be left with nobody to propagate. And then the inbreeding danger raises its ugly head. The genepool becomes stagnant.
I still say that Aizen didn't want to mess with Unohana because somehow he knows that Unohana is strong. Hell, Hitsugaya was a combat oriented shinigami and he took him out in one shot. To Unohana, he calmly told her the truth but did not instantly kill her. I mean, why shouldn't he kill her? If he didn't kill her, wouldn't Unohana chase after Aizen or worse, get some other captains to go after Aizen's head?
My conclusion is that Aizen couldn't risk fighting Unohana as he was not sure he could win, or at least win fast like against Hitsugaya and had to get to Rukia ASAP to do whatever he wants to do with her fast before Unohana catches up.
So to conclude: AIZEN IS AFRAID OF 4TH SQUAD! LONG LIVE THE 4TH SQUAD! EAT THAT ZARAKI! THE BIGGEST BADDIE IN BLEACH IS AFRAID OF A 4TH SQUAD CAPTAIN, and a woman to boot.
Just my opinion, I'm not saying I'm right, just my observation.