Why was there ever any expectation for Swift having good platform support outside Apple? This should have been (and was to me) already obvious when they originally announced moving to Swift.
Apple puts zero resources into making that claim reality, however.
But in the end I can't help but feel Swift has become an absolute beast of a multi-paradigm language with even worse compile times than Rust or C++ for dubious ergonomics gains.
Does it make sense compared to C#, Go, Rust or a JVM language? I don't know, but it's there, and Apple put some resources behind the initiative.
And if you mean before the whole .NET Core rewrite, many big corp server applications were already written in .NET, products like SharePoint, Sitecore, Optimizely, Dynamics 365, Four51,.... and plenty of server infrastructure for XBox and Windows games.
Have you actually used .NET on Linux/macOS? I have (both at home and work) and there isn't anything that made me think it was neglected on those platforms. Everything just works™
[1] Swift was first created in 2010, 16 years ago and officially launched in 2014, 12 years ago, by a huge multi-billion dollar corporation. The same corporation has launched hundreds of products and services in the same time period. If they really wanted Swift to be cross platform at a competitive level (Java, .Net, Python, JS, etc), they would have done it by now.