Monero has it's own privacy scheme, and by default, and as a whole will still be more private than ETH with zkSnarks. Privacy in monero is embeded into every address, not something added as a feature when necessary, which makes it harder to trace than zkSnarks on ETH theoretically.


Dash could eventually implement a zero protocol like PIVX, adding to it's anonymity, not sure how that would compare to zksnarks on eth.  Dash isn't focused on being a private coin, just a currency that has privacy features.

I don't really think it will effect either of them, they still have their niches.