Some people think he just left so that bitcoin wasn't under his control/guidance and it could left to be truly decentralized. Personally I think he died.. I don't see how any human could leave their wallets untouched if they had that many bitcoin in. Unless he/she was already wealthy 
if he/they was already wealthy?
after death someone inherits something. it is as a rule.
I'm unsure about the group theory. Every extra person is another weak link in keeping silent and with that amount of wealth available I don't see how every single person of what might be a large number would be that altruistic.
How many businesses and corporations were built on the wave of Bitcoin development? perhaps its not just altruism, but the quality and ways of enrichment.