Yes, steemit has not benefitted from PoB (though some of the founders have through their company-granted accounts). But as you note it has declined greatly due to selling (and being given to new user accounts before they stopped doing that), and that would have happened regardless of PoB. The same can be said for many other large accounts which have sold a lot.
Most of the other large stakeholders (including, as noted, most of the Steemit founder accounts) have gained enormously from PoB especially since bidbots became dominant, but also previously due to n^2.
IMO the most effective route to decentralization has been selling, and not PoB.
Well, you're right about that. Decentralization has definitely been driven by Steemit, Inc being forced to sell to cover development costs. This is why I'm happy they're selling as long as they are not in danger of going belly up because of the price dropping too low.