Well I can only speak to my personal experience, I haven't had any of my Photoshop or Illustrator objects that couldn't be read in Gimp and Inkscape. Gimp can also export to Photoshop format among many others, so I'm not sure whether there's really an issue with some specific formatting that just doesn't transform properly or something, but the capability is there in the tool and I've never had any issues creating photoshop files from gimp for someone who had to have that format, or tiff, or any number of other formats.
Sometimes people think gimp can't save photoshop format because they are looking for that in the "save as" area, it's not there, it's in the "export as" menu.
I think you might be underestimating how many people actually recognize Gimp/Inkscape as a valid set of tools to use for professional graphic design, there are plenty that use these tools professionally and often people will be happy to accept a wide range of formats even if they default to asking for photoshop first. Most often in my work there has been at least some format or another that can be acceptable.
Generally, linux open source software that are replacements for popular paid software almost always gets created with conversion utility to read the common formats whether vendors like Adobe like it or not, often the tools can also write back out into those formats too.
I do have an example where I can't write back to a format so these do exist on some packages. I tried to use LibreOffice as a replacement for Microsoft Visio, and even though it can read all the visios I've ever tried, it can't write back the changes in visio format. Since my company standardized on visio for diagrams I'm pretty much forced to keep a windows instance around just for Visio.