Personally, modifying a whole complex app like Dolphin to be modular, only to get some parts of it used inside open and save dialogs, is not a good decision to be made.
Unless those shared parts will surely undergo a constant and heavy changes in the future.
In X11 you can embed (“reparent”) a 3rd-party program in your window, without requiring it to provide an “element” (e.g. a DolphinPart). This was how browsers used to play video files in Linux a decade ago (embedding mplayer). But they now use plugins.
Yes, the Dolphin part would work. But I feel those code-sharing with KIO and dialogs and DolphinPart, is the reason that Dolphin is the most complex file manager implementation I’ve ever seen…
The file picker thing drives me NUTS. In KDE Plasma, it is a pain to pick an image file. In Dolphin there is a slider to enlarge the thumbnail, but nothing in the popup picker dialog. The file picker dialog is almost useless for picking an image. I have to copy/ paste from my regular file manager or click on each and every image to get a preview. Am I supposed to remember the cryptic filenames associated with more than 12,000 images I have on disk?