This is assuming that vote farming has 0 impact on the market value of the network. Which can be true temporarily, but is unlikely to be true for long. The market value of the network has far more impact on your net value as a large stakeholder than attempts to increase or maintain a % share.
The largest stakeholders don't actually have to trust that smaller stakeholders will not vote farm. They have the power to enforce it now, although I do agree that power could and should be made less costly (IMO the major costs are either human time or network resources, however Steemit Inc has not even really tried. @dan did in the past, and he tried to make it easier to enforce)