The OP defeats his own argument. LSP was a collaborative effort that benefited from a degree of coordination that only hierarchical organizations can provide, yet it still sucks ass.

OP blames FOSS for not providing an IDE protocol a decade earlier, but doesn't ask the rather obvious question of why language-specific tooling is not only still around, but as market-viable as ever. I'd argue it's because what LSP tries to do is just stupid to begin with, or at least exceptionally hard to get right. All of the best language tooling I've used is ad-hoc and tailored to the specific strengths of a single language. LSP makes the same mistake Microsoft made with UWP: trying to cram the same peg into every hole.

Meanwhile, Microsoft still develops their proprietary Intellisense stuff because it actually works. They competed with themselves and won.

(Minor edit: I forgot that MS alone didn't standardize LSP.)