I'm all for open toolchains, but I don't think this is the primary reason for FPGAs lack of broader appeal. I think the lack of open toolchains and the lack of broad appeal both stem from FPGA vendors almost exclusive focus on high margin, high-end applications. There doesn't seem to be much push to focus on larger markets with lower margins.
That said, Lattice has recently started to push into these new areas, but they haven't been that successful. If they start to see more success, I think we will see open toolchains. Lattice also has the advantage of being able to lean on the open toolchain work done by people like Clifford Wolf [1]