Besides Typescript, I haven't seen any language server do better in syntax completion and introspection better than the JetBrains equivalent. They probably don't want to work on it because 1. If they write their own they can make it work to exactly what they need, and 2. They would probably not leverage as much working on the OSS version as they'd lose by open sourcing and doing the bulk of the work on what makes them money. That might sound cold but it is not unreasonable.