Q: But what about existing open source tools (inkscape, gimp, krita...)?

While I'm very greatful for their work and what they've brought to the community, I personally couldn't get used to them. They feel slow, outdated and are not made to work together elegantly. I also have friends that are artists that feel the same way.

While I do believe that these tools work great for some people, they do not work great for everyone and a little bit of choice never hurt the open source community.