To add my voice, please, please, pleeeeeaaase consider a Linux version or a Kickstarter to fund the development of one. I have already purchased Affinity Designer (and Photo) for Mac, Affinity Designer for Windows, and I would buy it again for Linux in a heartbeat having made the OS switch.
I came to this thread via trying to troubleshoot an error while attempting to install Designer via Wine.
Affinity is literally the only software keeping me attached to Win/Mac in any way at this point and I'm confident there are many others like me. As others mentioned, there are things like http://snapcraft.io/and http://appimage.org/ to make life easier. Linux might be a smaller share than Mac/Windows, but you'd also completely dominate the space.