The idea is that users would converge on a consensus Schelling point through various communication channels because of the overwhelming economic incentive to do so. The situation in a BU world would be no different than now except that there would be no reliance on Core (or XT) to determine from on high what the options are. BU rejects the idea that it is the job of Core (or XT, or BU) developers to govern policy on consensus or restrict the conveniently available policy options on blocksize.
There is no mechanism to reach consensus about the blocksize limit, with BU all nodes just set there own limits. This does not reduce the potential for debate, arguments or central planning. The community would still need to reach consensus about the blocksize limit in the same way it is trying to now. The core development team could still try to keep the 1MB limit and miners would still need to make a decision to change to accept larger blocks, the only difference would be the debate may be about values to set in the client, rather than which client to use, which is effectively no significant change from the current status quo. If we want nodes to dynamically set a blocksize limit, in a way determined by the market, we should use a proposal like BIP100. BIP100 actually allows miners to dynamically set a blocksize limit and agree with each other on a new limit, BU has no system to enable nodes to agree on the limit.