Can't say that I agree, I've got a little over 30K LOC app running in AWS Lambda and Docker containers. I do all of my development on a Mac. Haven't touched Windows in over a year.
I've got a headless UDP multicast networking app in F# that runs just fine on Windows, macOS, and Linux. If that's not miles from Microsoft's traditional core app market, I don't know what is.
Can you clarify what you mean by not being supported very well? F# has been fully supported on .NET Core for a while: it's a part of the .NET SDK, FSI support is in, it's fully tooled in Visual Studio, etc. Would love to know what you feel is missing here.
I suppose language wise the support is there but there are lack of libraries. A while ago I was consider considering F# for the backend. While looking for database client I found most third party like SQLprovider, rezoom.sql didnt support .net core very well.
SQLProvider has been ported to .NET Core - I've used it no problem when scripting/prototyping across a variety of different data sources at once. Admittedly I tend to just use straight ADO.NET anyway - its simple enough to use and with F# your not saving all that much code/if at all moving to Dapper since the language tends to be more succinct.