The reason they didn't use GIMP is because they couldn't use GIMP. It simply didn't have necessary capability. When I was working in prepress, I would have done anything to use GIMP. That desperation to escape Photoshop is why Affinity took off.
If Inkscape could get a UI for precision positioning, something you could e.g. design an entry form in; and Scribus could polish up, I think a lot of people would move to a FOSS workflow.
I guess - of course it's a chicken-egg sort of a problem. No one's going to use it for print, before it has print-related capabilities - the same can be said for much of the UX, in general.