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.