I feel like off-chain ZK applications are really undervaluing the potential of just using STARKs, including for privacy-demanding use cases.

These days a STARK is < 100 kB, for off-chain use cases that amount of  bandwidth is utterly trivial compared to the client-side prover time savings that come from using STARKs.

What are you comparing the zk-STARK to here?

What do u think are prevailing privacy demanding use case examples?

For a lot of offchain applications you can also get away with interactive proofs