This would make the design more consistent with modal dialogs of Kirigami stuff, such as the one used to change SDDM background, for example. It would also fix https://bugs.kde.org/show_bug.cgi?id=437653, maybe?
These dialogs you're thinking of are implemented using Kirigami.OverlaySheet, which isn't designed for a UI as large as this. In fact, these dialogs use Kirigami.OverlaySheets inside themselves! The only UI change we could consider here is going in the opposite direction and make the dialogs into pages, so that they seem integrated into the main window itself. System Monitor already does that, and we could conceivably do the same thing for the GHNS dialogs in QtQuick KCMs. I wouldn't be against it. However we couldn't use this style everywhere; for example it wouldn't for the dialog that you can access from the Widget Explorer.
> The only UI change we could consider here is going in the opposite direction and make the dialogs into pages, so that they seem integrated into the main window itself. It's a great solution.
This would also fix that issue where the search box is hidden in a ellipsis menu.
Just to weigh in on my own here, this is actually the intention for the medium/long term - basically, when i originally built all this, both the KNSQuick Button and Dialog controls were always intended effectively as porting aides, since that method of interaction (as this report also suggests) basically just doesn't fit into what our UI is evolving into. So, from my point of view: Yes please, definitely do that :) (as Nate says, of course, this wouldn't work everywhere, but in the places where it /would/ work, definitely what we want to try to go for)