> BTC + Privacy is the holy-grail of crypto

The holy grail doesn't exist. It's all about trade-offs.

Bitcoin has a transparently auditable supply. Privacy coins based on Confidential Transactions have a choice between perfect soundness and perfect privacy. So far they all choose the latter (Pedersen commitments), as it also happens to be more efficient than the former (El-Gamal commitments). But it means that knowledge of a single discrete log allows undetectable inflation.

Then there's the scalability trade-off. Bitcoin allows a full client to prune all spent outputs after verification, to minimize disk-space. Privacy coins need to keep some information about past transactions. Worse still, the Initial Block Download is way bigger due to the sizeable range proofs. Except for the Mimblewimble based coins, in which spent outputs can be completely forgotten. But those are still mostly traceable...

Then there are trade-offs in emission. A fixed supply sounds great for scarcity/speculation, but also risks mining instability / security risks down the road as the block subsidy dwindles.

Then there are trade-offs in transaction throughput. You can easily increase it with bigger and more frequent blocks, but then resources needed to fully verify all transaction history will make it less decentralized.

A quickly falling block reward is good for early adopters but also leads to wealth concentration.

Smart contracts increase functionality but as they say complexity is the enemy of verification, and the attack surface gets much bigger.

Etcetera, etcetera.