Tbh I don’t understand your answers for both of the questions?
answer: “Shared resources will just brake in future.” (I will choose old UI, when options are: old UI / broken UI)
How? Why? And why an external resource pack will be better?
answer: “Yes. but … plugin writer may not be available any more.”
Yeah but, then someone else can update it, if there is a plugin in external resource pack and it breaks, someone needs to fix it too. If there is no original plugin developer, someone else needs to do it or the plugin will stay broken. It’s the same with default plugins too.
generic collection of UI icons
See Symbols library for Krita's mockups (more/new contributors welcome!)
dialog templates .ui (info-, warning-, progress-, about-, … dialog)
Qt already has info and warning dialog boxes by default. Progress could be useful, I guess. About could show the manual.html?