Is Bitcoin the final form for humanity?
No, Bitcoin is just a history of humanity, humanity will experience and pass the era of Bitcoin when it found a better form of technology. The world is everchanging, just when people thought that going to space is impossible, your thought of Bitcoin as the final for for humanity is answerable by no.
Decentralization has long been dreamed by all different social models, is there another step to be taken?
And it will be stay as a dream until people don't need government to centralized all things.
Can humanity (humans) learn to hang on and manage their finances?
Humans are capable of learning, there are already people who are capable of managing their finances. There are even lectures and books and how to do that. I do not know why you think human are incapable of such, the world is to huge, we have to look out of our window in order to see some of them better yet to explore the vastness of the world.
Or is Bitcoin an experiment to show that we're not capable of managing our own belongings and do require central authority?
I do not know what are you trying to convey here, there are people who offer to manage other people's belongings so it means people are capable it is that many are too lazy to do or learn it.