Very narrow one sided view there... The "barely used nowdays" and "specific areas" are leaving sideways an industry that surpasses Hollywood: Videogames
Reminds me of the comment "Nobody plays single player games anymore". Careful with what you say or you might lose customers. I highly suggest to add that export functionality or you might lose the "very specific areas" customers to other software out there plus the bad PR that that might bring. In all honesty I only work with targa files, and I can't really recommend Affinity to my colleagues if it will make their work harder and they are itching to ditch photoshop.
We are loyal to whoever or whatever makes our work easier; Having to use a third party application to convert the files to targa is adding more work to my process making me slower. Please, adding that file format should be a piece of cake for your devs.