I think you have a great idea here, but it wont solve the centralized problem of steemit.
"This would make the Steem platform just another reflection of the pyramid/hierarchic system we see offline where the big banks control most societal activity to some extent - this is the opposite of decentralisation."
What is the reason for steemit being centralized. Is it the bot's consolidating the steem or the fact that the more you have the more you can make. This is the truth in the real world also and what I think is centralizing steemit is the more you have the more you can have.
It is true that whoever has the most steem is able to centralise power in their own hands - however, by shifting the control of bots from random people into a form that takes the power back to the general steem pool - there is less centralisation in those specific hands. At least then the only way to centralise would be to actually bring value in the form of quality content or through buying steem directly - both of which actually increase the value of steem and so there is a form of value for value exchange occurring which cannot occur another way.
This is what I thought was a good idea
"however, by shifting the control of bots from random people into a form that takes the power back to the general steem pool"
It does solve very well the problem with the bots.