It definitely looks like a cool app, and I was excited to test it out, but I don't have a Mac. If you ever hit the point where a rewrite makes sense, it would be awesome as a universal app.
One idea I've been toying with would be to do a Kickstarter-style campaign and if it reaches a certain threshold, then I know it would be worth porting.
Targeting Windows/Linux/web still means I cannot re-use the sources. But targeting web might be faster in terms of development time, although I don't have deep expertise on non-Apple platforms, so I cannot say for sure.
Why? There are already all kinds of web sites that do this kind of thing. Monodraw's unique selling point is that it's a native Mac app that takes advantage of the Mac UI and it's done well so the UX is top notch.
If you don't care about making the best possible app that you can, go ahead and do it in the browser. You will get something that's probably good enough and runs everywhere. But it's going to use more battery, more memory, and more bandwidth and not feel like a Mac app. Plus (IMHO) it's less fun to develop for the browser.
I wish I had the time to port it to all three desktop OSes.