Any replacement for these will basically have to be a bug for bug clone if you want them to work. LibreOffice is 80% of the way there, but it still mucks up too often to be reliable. PDF viewers are plenty, but there's no effective replacement for Acrobat, InDesign, Illustrator and Photoshop[0].
Third party vendors you have to work with for other things (ie. Printing folders) require stuff to be in the formats made by these two software suites and their response to "your printing press isn't following the PDF spec" isn't gonna be "oh sorry, we'll migrate our hardware", it's gonna be "the printer says no and my other customers don't complain so just send me the files correctly."
Since Adobe and Microsoft are the default, this is something third party vendors can say and get away with. The shoe is on your foot, not on theirs.
[0]: GIMP doesn't even come close to being a Photoshop replacement, they do very different things. Photoshop is a photo editor + drawing program, while GIMP is aimed at image manipulation. The difference comes into play with how the interface is designed and the complexity of certain actions in each program. GIMP is designed to let you do specific individual things to an image, while Photoshop is more aimed at giving the user entire workflows.