I think we can't find an agreement on our angles on the topic so much that it's simply not constructive to push the conversation further. I'm afraid that if I'll try to say anything now, it'll be a repetition of what was already written earlier.
In short, I see Slint as a not GPL project (but rather as a commercial project that happens for now to triple-license the code and includes GPL). I see GPL projects as fundamentally different to Slint, in a sense that, once you have enough external contributors, you simply cannot revert back and stop being a GPL project, whereas in Slint I see it as possible. I trust GPL projects and I know I can "lean" on them, whereas I'd advise to rely on Slint only if you have commercial entanglement that you want to keep.
I'd propose to agree to disagree.