"Premined" is not strictly accurate. 1/10 of the mining reward will eventually go toward a wallet controlled by the developers. The rate of the incentive payments starts out high and decreases over time. [1] This is different from a premine or instamine, where the devs begin life with a stash of protocol tokens, which they then have a strong inclination to dump on the market.
Zcash 10% vig is an improvement over instamine or premine. I agree this structure is still not ideal for maximizing zcash value, as it creates an incentive to clone zcash (without the vig). Zcash looks like it will be rather more difficult to clone than, say, Ethereum. But it will certainly be done. I also expect that some of zcash features will be added to other protocols, thus diluting its tech advantage.
No one has figured out how to force everyone to invest in the same version of bitcoin, and perhaps that is as it should be. I still expect the Zcash devs to make out OK.
[1]https://z.cash/blog/funding.html