[Chickenality]

I think this assumes that the value of the voting rights is constant. In OP's example, the stock buyback only happens for voting shares, which should cause the market to update its estimate of the value of voting rights.

[thirtyseven]

Good point! I don't really have enough (or any) economics knowledge to know how to model the value of voting shares in this case, but I think that there would also be downward pressure, for example as the voting power of company founders increases due to the reduction in number of voting shares.