I would say yes they could really replace a lot of artists, but artists can also use it to help finish their CG painting faster, or to get the fundamental of the piece for the larger work frame. The world is changing fast, we need to adapt and instead of being left out of the trend, try to think positively - maybe we can learn new tools and adopt them with our current skills.
Another thing is that they can't be made accurate enough to show emotion on the pieces (at least in my opinion) or match a client's specifications compared to what an artist has in mind. They might get very close to what we have in mind, but at the end of the day, they still can't directly read your mind, and even if the generation goes in a slightly different direction, the hours of modeling can be wasted and you have to do another rendering. So there is still some room for improvement - I know the technology is advancing fast and may soon be a lot closer to what we envision, but again it also depends on the artist/illustrator whose skills do the retouching of the image would affect piece.
What I think about this whole AI painting is - don't give up the traditional art! What you can do with the brain and hand will help your CG painting. What you can do in CG painting will help your enhance and edit your CG rendering paintings. The human brain is still developing.