This is an oldie, but I'll add my 2 cents.
The first point to consider is the ecosystem the designer lives in.
An obvious reason not to choose different applications is if it has a strong dependency on external assets. This is probably the main reason people do not choose another platform and this applies to a lot of things, for example, social media.
If the workflow is somehow closed, or you use general files types (Like PNG, JPG and TIF for photo images) you are fine using different programs.
Compatibility has increased a bit more after the years, and the usage of output formats like PDF is more solid now than a decade ago.
A specific scenario where is totally suitable to use Open Source programs is in Institutions, where you can have a "Communications Department", elementary schools, etc and you need to use the software on several computers.
My question is with regards to the possibility of fully divorcing from Adobe's solutions and still being able to function as a graphic design studio.
For a "professional" studio, Inkscape and Gimp are probably too limited even now. But I should say that it is totally suitable to divorce from Adobe. But again, depends on the workflow.
I use Corel Draw since forever and in some have some nicer features than AI. Even some adjustments that could be done on PS I do them inside Corel Draw.
I still have a copy of PS for very specific stuff, but I use it less than once a month.
The new kid on the block is Affinity Designer and Affinity Photo.
On the "Open Source" world, the most prominent program is Blender for animations, compositing, besides the obvious 3D.
On some other areas, like digital painting, there are a lot of alternatives rather than PS, like Corel Painter, Krita, Sai, etc.
It is better for everyone that we have competition. But in this interconnected world, it is becoming more difficult.