That still significantly raises the barrier to entry until someone writes a generic CSI implementation that translates to the flexvolume API.

And then it's still yet another daemon running on every kubernetes node 24/7.

Regardless, in our case the scripts are full-fledged python applications so inevitably we'll just turn them into grpc servers. We're an advanced kubernetes shop so this isn't a real hurdle, but not everyone is.

My complaint isn't that it is impossible, it's that it makes it much more work compared to the previous API and by selecting GRPC as the protocol instead of HTTP, they are largely disregarding the trivial language interoperability there once was for little gain. GRPC+protonuf is a PITA in non-go languages and build systems.