This seems to me a less imminent risk in comparison to the incentives that already exist in the ecosystem - which are not the ones most aligned with long-term sustainability, for which market cap growth is essential. Zcash will not survive and thrive unless it is a reasonable place to store value AND sustainable from a security/mining standpoint which won’t be the case if the price craters or flatlines. If Zcash does a 10x on the basis of purely self-interested behavior on the part of coinholders, everybody wins.
That said, the best system IMO is one with checks and balances, meaning some sort of system (several which have already been proposed) that treat coinholder votes as one decision making body subject to the veto of another that not weighted purely by the value of holdings.
My biggest concern with overweighting coinholder votes is that someone buys up or borrows a huge quantity of ZEC solely for the purpose of influencing the outcome of a vote to do something nefarious, like say weakening privacy features, as you describe above. A body comprised of stakeholders from across the system with veto power should be able to hold that in check.