Sketch, Rogue Amoeba apps, Panic, etc. plenty of Mac-only businesses. They make decent money. As Mac users actually tend to pay for their software rather than expecting everything to be free.

If you released the same software for the same price on Windows, not only would it be more difficult to develop (due to Windows lacking the same abstractions as macOS), you'd probably have way less users, as Windows users don't typically buy apps like this.

I share similar caution with tools and multi-platforms (specifically Mac, Win & Linux). However, the format is interchangeable with other Ascii drawing apps and it isn't too expensive. So I'm totally fine with it.