What do you mean by "Grin’s privacy depends on network activity" ? Grin's hiding of amount and address doesn't depend on that. Only the use of CoinSwap to hide the tx graph would depend on a sufficient amount of daily CoinSwap txs.
While Monero's privacy is a little better, it pays a large price for it: having a 20x faster growing chain, and the need to remember randomly accessible data from every output ever spent.
Its privacy isn’t default—linkability depends on high network usage. If volume drops, transactions become easier to trace. Monero doesn’t have this issue because every transaction is private by design, no matter what.
Yes, Monero’s blockchain is bigger, but that’s the price of real privacy. Grin stays lightweight by cutting corners on privacy guarantees. If privacy actually matters, Monero is the clear winner.
The extra transaction step adds robustness, letting the receiver state and sign for the purpose of payment (e.g. the item purchased or the service rendered).
Monero obscures the tx graph with decoys (effectively only 4 as recent research by Rucknium shows), which cuts corners compares to the much stronger privacy guarantees of Zcash's shielded txs.
The extra step isn’t robustness, it’s friction. Payments should be fire and forget, not a back and forth process. Monero works because it just sends, no hassle.
And Zcash? Please. Over 95 percent of its transactions aren’t shielded. What good is stronger privacy if almost nobody uses it? Monero may have only four effective decoys, but every single transaction is private by default. A privacy coin that relies on opt in privacy isn’t a privacy coin, it’s a compliance coin.
Ensuring the correct recipient and purpose of payment is not only robustness, but also added privacy since payjoins obscure the direction of payment, something Monero lacks.
With only 4 effective decoys, Monero's privacy is deficient, while Zcash shielded txs enjoy far superior privacy, independent of all the transparent txs.
Grin prioritizes scalability over privacy, which is fine if you don’t actually need strong privacy. But let’s not pretend optional CoinSwap is a real substitute for built-in, default anonymity. Privacy that depends on extra steps and high network usage is fragile by design.
Monero doesn’t need payjoins because every transaction is already private. And calling 4 effective decoys “deficient” ignores that Zcash’s superior privacy is irrelevant when 95% of transactions don’t even use it.
If privacy is optional, it’s just a feature. Monero makes it the foundation.