Unix != Linux. Different kernel. Since there were lots of distributions, we had AIX, FreeBSD, etc. These are Unix, and often the *nix is for Unix of all flavors. Linux is a variant, separate kernel, and perhaps people think *nix as Linux as well. I just wanted to be sure we were talking the same thing.
Ubunutu, RedHat/RHEL, CENTOS, Mint, SUSE, etc., all Linux. Only a few have SQL support, though I hope the list grows, at least to CENTOS.
Windows and SQL Server are so big, that even if 10% left, it's a huge number. But it's a small fraction. SQL Server will grow more in the Linux space. There are already lots of MS customers that use Windows desktops, and have a few Windows servers, often because of SQL Server. They can now just go straight Linux.
Really, I think the future is containers running in Kubernetes. Linux containers are just super lightweight, so I think that's the direction to move.