That's good because OpenGL improvements in Mesa for AMD, Nvidia, Intel aren't making those three big bucks. Gaming is making AMD and Nvidia money and Intel can afford to dump money down a rat hole slowly improving OpenGL for Mesa. They're competing for other markets.
Your 2019 is non-factual hogwash. Game Devs dictate when Mantle is ready for Linux and the more they move to Linux the more resources will be getting Mantle optimized for Non-Windows platforms.
Even OS X will help considerably getting Mantle ported more rapidly, now with AMD's expansion into Apple hardware.