But Slint is open source and you can for it if we were to change the license, and the royalty free license is free of charge.

Even MIT/Apache software can change license and start charging, as we've seen in the past with some project.

My app is proprietary though, so I can't use the royalty free license. I've definitely licensed commercial software before. For example, I've licensed OCR software for many years.

The difference to me is that with OCR I can change it out with acceptable effort in the case that the vendor does something that we can't agree to. But it's just too hard to swap out a UI library. That's a major rewrite. It's too risky, to the point I'd rather spend resources bringing an MIT/APACHE UI kit up to my needs than risk being locked in with a vendor.