I think "useless" is way too strong here. Swift doesn't have the same community support outside of Apple platforms as something like Rust and Go, but it's a very capable general purpose language with plenty of platform-agnostic packages available if you look for them. Also the C interop is great, so you have access to the last 50 years of prior art.
I think the argument is vastly overstated that Swift is not useful without Apple frameworks. It's similar to C# - you have the language, and then you have Windows APIs. Nobody would argue that C# is "useless" outside of Windows.
With the small detail that nowadays Microsoft is heavily commited to make C# and all the libraries that aren't using Windows specific APIs work with the same level of support across all platforms.
Compare Swift on Windows versus .NET (VB, C#, F#) on macOS.
Oh I totally agree that C# has a much better cross-platform story than Swift.
My only point is that that the argument I seem to see often that Swift is not useful outside of macOS/iOS because you don't have SwiftUI/CoreData/whatever is not a good argument.
Kind of, Swift doesn't exist in isolation, there is competition to think about and unless we are speaking about iOS devs using Linux servers, there is very little business value not to chose some other technology with a stronger cross-platform support.
Yep, it sure seems like professional adoption of Swift on alternative platforms is microscopic outside of people who were already iOS/macOS developers. I don't think this is what they had in mind when Lattner somewhat-seriously talked about the "World Domination Plan" in the earlier days.
Jony Ive - Introducing Swift Actors .......
It's a nice language but useless if you want to use it as a generic purpose tool.