Microsandbox claims to start faster than docker, and it is isolated from the host, and to work with OCI. Why would I still want to use docker? The only reason I can imagine is that I actually want to be able to dynamically share resources between containers instead of dividing up VMs a priori.

Ah, the significant compute overhead: https://josecastillolema.github.io/podman-wasm-libkrun/. Much more cpu and ram usage at worse performance.