Microsoft will never charge for usage of .NET simply because that's not their business model which relies on mainstream adoption, previously it was to spur sales of Windows Desktop, Servers & their Infrastructure software but now with Azure they're happy for it to have ubiquitous adoption on Linux or Windows servers as well.
However, this isn't something I'm really concerned about. I think there's practically nil chance something like that would ever happen.
This creeped me out just enough that I haven't played with it. Microsoft has changed a lot but downloading Linux binaries from them is still just a bit too much.
https://github.com/dotnet/fsharp#contributing
They even have a Visual Studio Online quickstart which provisions the build environment with all necessary dependencies:
https://github.com/dotnet/fsharp#visual-studio-online-quicks...
Where the build environment wouldn't be running on your machine.
Even compiling the compiler and then use that compiler to compile your code doesn't help, because the first compiler that was compiled by someone's else might tamper the resulting compiler binary.
So why are you still looking for issues when there's solutions avoiding their stated objection?
> Even compiling the compiler and then use that compiler to compile your code doesn't help
So what's the threat model now? that MS will risk their reputation, established business models and subject themselves to liability to poison the bootstrapped compiler to detect when a modified F# compiler is being built so they can inject vulnerabilities into all new modified F# compilers ensuring they're similarly poisoned whilst avoiding detection from easily disassembled byte code (that can be crossed checked against its original source), Does that sound plausible?
For a long time the F# community criticized Microsoft for their lackluster tooling support in VS where it was treated like a 2nd class citizen, but that's less of an issue now that a lot of devs are developing F# with cross-platform IDEs like VS Code and JetBrains Rider.