If you don't mind your app looking like shit on all platforms, this is a viable strategy. A lot of specialist applications are like this: for example, I used to develop Qt-based GUI tools for robotics.
If your app looking like shit will cost you significant sales -- as is the case for most productivity and design apps -- then yes, you have to port the GUI bits to each and every toolkit you're using.
Not true. Lot's of applications use generic abstractions and look good at the same time.
Firefox, LibreOffice, VirtualBox and etc. If you don't know how to make good GUI using generic toolkits it doesn't mean it's not possible.
Anyway, if you buy application for its "looks" - there is something seriously wrong already. It should look good, no doubt, but it should be functional first.
>as is the case for most productivity and design apps
Completely the opposite. They tend to provide completely custom, not native looking UIs, and therefore using cross platform toolkits for them only makes more sense.
Oh, yea. And Mac users are oh-so-great majority of desktop users... Please. I have nothing against Macs and when I'm rich enough I might even buy one, but I won't try to force the whole world to support my niche OS then. Or niche aesthetics for that matter.
If your app looking like shit will cost you significant sales -- as is the case for most productivity and design apps -- then yes, you have to port the GUI bits to each and every toolkit you're using.