I don’t think I am well positioned to answer that question given my experience. Ill give it my best.
I believe the advantage of more abstraction of gRPC was desireable because we can point it at a socket (Unix domain or internet sockets) and communicate across different domains. I think we are shooting for a “microserves” architecture but running it on one machine. FFI (IIRC) is more low level and more about language interoperability. gRPC would allow us to prototype stuff faster in other languages (like Python or go) and optimize to rust if it became a bottleneck.
Short answer is, we are able to deliver more value, quicker, to customers (I guess). But I don’t know much about FFI. Perhaps you can offer some reasons and use cases for it?
simply put, programming is glorified automation. There are jobs where the process that needs automating makes money.