I didn’t bat an eyelid when I bought Little Snitch, Alfred, Parallels, et al. and from the screenshots this looks like a nicely polished piece of software.

Hmmm, I don't like that statement.

"Great software" is very subjective. I think linux is a great piece of software. I use a macbook for the great hardware but certainly not for the software! Apart from Sublime text all apps I use are free, including hundreds of CLI apps.

I like Unix, which is why I use Linux and macOS. Linux because macOS is not an option and work, and I dislike non Unix systems like Windows. I like to have access to a powerful CLI.

But lets face it. Once you step outside the CLI on Linux, it compares very poorly to macOS. Not only do you have far more choices for every type of app but most of those choices tend to be higher quality as well.

Now let me qualify what I mean by quality. Many Linux apps does in fact have a significant amount of functionality. You got quite extensive packages such as Gimp and Inkwell. However these frequently have poorly designed UIs or UIs which are completely different from other applications, and frequently integrate poorly with the rest of the OS.

You mentioned Sublime. Well guess what that came directly as a reaction to TextMate on macOS. There are dozens of these types of modern incarnations of vim and emacs on Mac. Fortunately now we have started getting some selection on Linux with sublime and atom. But it was kind of slow, and while both have quite good core functionality, they are a bit lacking in UI. You notice how a lot of successful apps on Linux have to rely on HTML based interfaces. Which means you kind of notice the UI is kind of slim. Last time I checked neither Sublime and Atom had any real UI for creating and modifying bundles/plugins.

So some software is worth paying for, we're just arguing over which?

If you think the benefit of using the software is $30 or greater, like anything else in the world.