What important compromises are being made that aren't "libraries may not exist", which is true of any of the equally niche languages that get a lot more love because they aren't associated with Apple? Even the severely limited ecosystem is mitigated by the language having excellent C and decent-enough C++ interop.
Microsoft is discovering how hard it is to take .NET outside Windows, the fierce competition of UNIX first programming language ecosystems, and Apple is not doing with Swift half as much as Microsoft is trying to.
Getting started on C# and Windows App SDK sucks a lot for example, because everything you google about them returns boatloads of results for myriad incarnations of C#, .NET, and XAML-based UI frameworks which makes figuring it all out somewhat painful.
I bet very few of these work on Linux, https://swiftpackageindex.com/
ASP.NET, EF, Avalonia, Uno, Akka.NET, Silk.NET, Unity, Godot and tons of other software packages work just fine on macOS and Linux, with JetBrains Rider and VSCode/C# Dev Kit/Ionide.
And despite all of this, Swift is a toy on Linux compared with .NET ecosystem on Linux.
Again, newer languages having less of an ecosystem than older languages is not news or somehow unique to Apple (or Microsoft by that matter). The existence of more mature languages than whichever one you're considering is not news - if it was, then nobody would start any project in anything other than like three languages. And as I mentioned there is a gigantic ecosystem that Swift and several others have access to by design, which for some reason people refuse to acknowledge whenever they shut down newer languages with "but no ecosystem".
> Microsoft is discovering how hard it is to take .NET outside Windows
Ten years ago you would have had a point. However it's 2024 and .NET is solid, stable, and widely deployed on Linux et al - and that's without taking projects like Unity into account. That's far from only working well in a vacuum.
It doesn't matter how solid and stable it happens to be, those widely deployed on Linux workloads are mostly from Microsoft enterprise shops cutting down on server costs, not startups filled with macOS, FreeBSD and GNU/Linux desktops deciding that C# would be a great language to use on their startup.
Unity is a special snowflake, they have their own .NET infrastructure, which is also a reason why you can't just use .NET vlatest on Unity.
Going back to Swift, so far it hasn't shown to be any different than open source Objective-C, and Objective-C at least has GNUStep outside NeXT/Apple.
I have colleagues who worked in a "macOS + Rider and Linux Hosts" style teams for a long time, and had great experience, but when they talk about this to a wider audience they get a similar treatment to what another commenter described when dealing with an architect from Walmart. People only ever listen to what confirms their beliefs and at best ignore when being exposed to something different, or sometimes react in an outright hostile manner.