It doesn't even have to be proper open source - just source open! Just dump the source and say people may not sell it, but they may study it and contribute back

It doesn't even have to be proper open source - just source open! Just dump the source and say people may not sell it, but they may study it and contribute back. You're not loosing any income - honest people will continue to buy the license, and those who are not honest or cannot afford it will continue to not buy a license. But people will be put at ease about the bus factor. We will learn a lot about how to produce a snappy app with a multiplatform toolkit. And I'm sure people will contribute back great features.