With all of the enhancements to C# over the last decade, is this still relevant in 2021? None of the benefits listed seem that compelling. (Serious question, not trolling.)
Extra keywords and extra files are annoyances that I try to instinctively minimise. I don't want to say I take the path of "least annoyance", but it's definitely one of the factors that strongly influence how I write code. F#'s lean tagged union type syntax is functionally inferior to Scala 2's case classes, but I use sum types more in F#, because they're less annoying. In C#, I might as well use classes, because I can't declare free functions.
I often hear that Java is good for teams, because it railroads programmers into writing maintainable, pure OOP code. Well, F# railroads you into writing data-driven, mostly functional, multi-paradigm code, and I think the result is much nicer code than what you'd get when trying that in most other languages.
If I had them as a first-class language feature, would I use them more? What for?
For example, in my biggest F# project I parse a local standard for public transport timetables. In that format a trip can either drive through a stop, drive around it or stop there at a particular time. With a sum type, I can represent all three of these cases without worrying about a value of that type having both "driving through" and a time set. When you've got a language that effortlessly allows something like this, you'll find uses for these paradigms.
1. Allows you to make failure scenarios intrinsic to your domain, and the compiler then is a tool for domain correctness.
2. It provides a beautiful symmetry between how you define your domain and how you operate on it. The syntax is even almost identical. The features "fit" incredibly well because of that.
Beyond that, with optionals and results defined as sum types you find that any time you're in a "It may explicitly give me nothing" or "It may explicitly give me an error" scenario these types are the first your reach for in your toolbag.
Sum types don't completely obviate classes, but they are a better tool than classes for several scenarios.
I think from the syntax, strong typing, and algebraic type signatures F# is the most information dense language I have come across, while still allowing for readability. Maybe something like brainfuck is more information dense.