Beyond the excuse to be a useful "dogfood project" of the language, was there any other intentional reason to focus on a custom IDE rather than the Language Server Protocol and multi-IDE support?
A LSP didn't give me enough control over the IDE environment to create the development experience I wanted. I was also very interested in building a very good hot code swapping environment and that's not possible with a generic IDE/LSP.
An LSP implementation can be fairly useful although if you want to let backend services like sourcegraph code search support your language better. Not to mention other future tooling or doing development with a beefy datacenter machine to have fast compiles or work with large codebases.
I would suggest adding LSP support as a future todo.
https://langserver.org/