I think you misunderstood the purpose of the CRaSH integration in Boot. Boot provides an integration with CRaSH so that you can connect to a running application and query management or other information remotely. The boot integration also provides custom commands to be able to query the actuator endpoints that are available.
Spring Shell is also an interactive shell that can be easily extended with command but it does not serve the same purpose at all: CRaSH allows you to interact with the VM and the "application" on which it as been installed while spring shell allows you to build your own shell with your own command for a "command-line" app.