> big bag of stuff, with no direction.

also called general purpose, general style langue

> that still can't really be done in C#

I would think about it more as them including features other more general purpose languages with a "general" style have adopted then "migrating F# features into C#, as you have mentioned there are major differences between how C# and F# do discriminated sum types.

I.e. it look more like it got inspired by it's competition like e.g. Java (via. sealed interface), Rust (via. enum), TypeScript (via structural typing & literal types) etc.

> Was a functional approach really so 'difficult'?

it was never difficult to use

but it was very different in most aspects

which makes it difficult to push, sell, adapt etc.

that the maybe most wide used functional language (Haskel) has a very bad reputation about being unnecessary complicated and obscure to use with a lot of CS-terminology/pseudo-elitism gate keeping doesn't exactly help. (Also to be clear I'm not saying it has this properties, but it has the reputation, or at least had that reputation for a long time)