If I had to guess, I'd say the trends will still be set by big design agencies and artists who get paid handsome sums to work on a big company's rebranding proj

If I had to guess, I'd say the trends will still be set by big design agencies and artists who get paid handsome sums to work on a big company's rebranding project. The difference will be that many of the smaller illustration gigs, which essentially boil down to "copy the style of this big company" will cease to exist. Those types of gigs don't require a whole lot of creativity or skill, because the client simply wants a knock-off of what someone else came up with, but for cheap. AI-generated "art" will allow those clients to bypass the human and just get what they want directly from a computer. In a sense, it's going to be similar to what professional photography has gone through in the past few decades with cameras becoming more and more commonplace and affordable. Some people still want a professional portrait taken of them, but the majority of people are perfectly happy with what a mobile phone camera can produce.