the actual next thing used is c++ though for large customers on big iron with Unix/Linux/Mainframe, never seen C# used server side.
I think you mean you've never seen it used server-side in a *nux environment right? Why does that matter? In a micro services architecture, you mix and match VM's running Windows, *nix, whatever. As long as they can communicate with each other. This is especially easy with cloud virtualization. I have worked at many companies even very large, publicly traded ones that are using an eco system like and in some cases supporting millions of concurrent users.
Why even draw the comparison to mainframe usage? Most companies are going away from things like IBM because it's too damn expensive for maintenance contracts. All IBM did was get vendor lock in. Their products are inferior in today's day and age. The only reason companies stick with them is because the cost of the conversion is insane. I've replaced IBM solutions before. I replaced an enterprise system running on an AS400 with a distributed message oriented middleware system written in C# and it had 99.999% uptime, was completely fault tolerant and summarily outperformed it with significantly less powerful and expensive hardware. I think you're living in the past. Don't get me wrong, it was good times but every new beginning comes from some other beginning's end.