I suspect an organic middle-ground is what most best-cases end up with. One where the manager has veto power, rather than dictatorial power: the employee finds something they feel is their "most valuable contribution" and the manager says, "Yes, that's good. Work on that" or "No, we don't need that" or "Maybe later, but your current focus is this other thing" or "Think about it more and figure out how line it up with our company vision better."
I've found that a number of my bosses have been quite understanding about this sort of thing. (I'm terribly lucky.) I have been able to pitch a new idea and get the nod to work on it. Granted, I've also never done the singular focus thing; I'm still required to pitch in with bug work and sometimes I'm the right person to ask for some random but important distraction.
Open allocation is a middle ground. It's not a free-for-all. People are still expected to lead or follow, and still held responsible for working toward the benefit of the group. What you don't have with OA are those imbecilic internal headcount limits and transfer blocks. You're getting rid of an often useless and sometimes extortionate layer of indirection.
I've found that a number of my bosses have been quite understanding about this sort of thing. (I'm terribly lucky.) I have been able to pitch a new idea and get the nod to work on it. Granted, I've also never done the singular focus thing; I'm still required to pitch in with bug work and sometimes I'm the right person to ask for some random but important distraction.