Gimp is no replacement for Photoshop and Inkscape is not replacement for Illustrator, ...
Btw, mostly everything you install with `apt-get` can be installed with `brew` as well (MongoDB, Elastic, nvm, etc.).
Nor is Photoshop a replacement for the Gimp, or Illustrator a replacement for Inkscape. They are not cheap clones forever playing catchup to the commercial packages, but mature tools in their own right. Sure, the FOSS tools might miss some of the advanced features the proprietary tools have, but for web development both of them offer plenty of features — and they are free to boot!
Whether or not one can replace the other is mostly a matter of what you are used to. No alternative is going to satisfy you if you've intensively used Photoshop for a decade.
I thought .psd mockups and HTML slicing were a thing of the past, and vector work (logos, etc) should be done by a graphic designer. Not to mention most icon/graphic assets can be provided by Bootstrap or FontAwesome.
No vector logos yet, but if they give me a .svg I can view it directly into the browser.
A much better example would be the native clients for GitHub or tools like those. I use the web ones and I'm happy with them.