LSPs are great, I think they've proven fairly easy to integrate into many text editors. But consider something like the Scratch programming language. How many editors support Scratch? Once you stray from code-as-text, adding support to old editors often becomes infeasible and the effort needed to create new editors is a significant barrier to entry.
I'm not familiar enough with it to know how much is truely in the protocol vs what the editor still has to do themselves.