>I think the reasons are quite clear: introducing asynchronicity (one requiring further user interaction at that) into what used to be synchronous code is one of the most common ways to introduce bugs.

I don't think it is that clear. If your app uses GTK or QT, you should in theory get transparent and automatic support for portals. I suspect the problem is the app then tries to directly access the FS outside of the filepicker and gets blocked.