IMO the hard problem here is PoS consensus with the private transactions. It seems like the stakers have to come up from the depths of privacy to participate in consensus. Maybe there is a way to do private staking, but that makes the network very difficult to understand and build confidence in. So I don't see upgrading to faster consensus to be a small incremental improvement, it's fundamental.

A separate issue is that both Monero and ZCash are not post-quantum secure, while many of the new zkSTARK VMs are. The ledger lives forever, and state actors will eventually decrypt the transactions if the cryptography can be broken. At that point it seems like it's better just to build the currency product in one of the zk VMs.