I will give you an example from recent weeks. I am a developer and I implemented a android app, all with AI, because Kotlin is not my language. But to make it working my expertise and skills to analyze, think of the architecture, security, databases, issues, debugging helped me make a stable product and I am really proud of the results. I got so confident that I thought I can do the marketing campaign with the same tools and success... AND I FAILED!!
Why? Because I don't know the basis, I can't decide what AI generated suggestion is good and what bad, what will work, and what won't work. Sometimes AI suggests things that are just not ethical. If I had a good basis, like in software development, I would have better results.
Yes, for sure, work will drop and it will affect some people, but web agencies will stay.
I will give you another example. I have all the tools at home to make my own desk, but at the end of the day, I go to IKEA and buy a new from there ;) . I believe that the number of people needed to do a job will decrease, and the demanded quality will increase, because manual labor is more expensive. Because it is more expensive, it make sense clients to demand more for what they pay.
Time will show :)