Literal ZBTC would need zcash, but you can build a version on Tezo or Ethereum that offers the same privacy and functionality. And people will. Tezos is planning on doing it using a modified version of sapling.
I think @balajis hit the nail on the head. Wrapped BTC isn’t risk free and never will be. So the whale Zooko has in mind who only wants wrapped private BTC (if they exist) will jump ship either to ZBTC or eth-z-btc. But there is a compelling reason for holding a native fixed supply asset that is private and those people will hold ZEC as BTC with privacy. And whatever that demand is, it certainly isn’t decreased by the existence of other assets on chain. Just look at what happened to ethereum.