>I wonder why there aren't other cross platform sketch equivalent.

Adobe XD, InVision Studio (soon on Windows), Figma (in browser), Framer (soon on Windows).

There exist a great one, which is called Figma. They use electron in combination with WebAssembly to make it fast. I'd say that's a pretty decent choice.

jumping straight into webassembly! very brave ... or is it actually more mature (production-mature?) than I thought?

Well, the support is pretty decent in evergreen browsers (and it already was for a while), and in case of electron, you have precise control over your runtime, so it's not really a problem. Also many engines have already done quite substantial optimizations for handling wasm modules in combination with JavaScript. It's all done with backward compatibility in mind, so what works today will continue to do so.

I use Inkscape on my mac all the time, though I have to build it with homebrew to make it work nicely.

edit - though it did get borked by a brew update and I had to add a different source to brew to get it back up, so is not that sane admittedly.

Write your core code in C++ (or Rust, if you so choose), and write a thin UI wrapper around it using a native API.

If budget is not an issue, I would use Qt with C++.

Why did you specifically talk about budget? Are cpp programmers expensive? What would be cheaper?

Qt license is USD 5500 per user per year. For Sketch developer, this should not be an issue. But for some developers who bootstrap the cross-platform application, this might be an issue.

Also, C++ programmer is more expensive.

The cheaper option would be JavaFX. IntelliJ IDEA is one of good cross-platform application examples.

Thanks, I don't agree with the "good" part of intellij idea. Popular, sure. But slow as hell and ram hungry.

Inkscape (built with gtkmm)? sk1?