I guess as always with progress its about effectivity: Sure you can draw everything on a piece of paper, its just more efficient to use readily available brushes and the many tools a software offers you instead. Here in this case, the workflow will be a close mixture of AI and manuell edits to take the best of both worlds - but for that you need seamless integration.
And with that type of software, the job of a grafic designer will change drastically: Have a look at https://www.instagram.com/explore/tags/midjourney/?hl=de : These are all pictures created by "amateurs" with the help of AI - why would I pay a designer for that if AI can do it better, faster? These AI models were released only 4 weeks ago and already have huge impact - just imagine 1 year of improvements and integration into workflows...
AI grafic design is there, it will be HEAVILY used to create/modify pictures, and without an plugin interface, Affinity Photo will totally miss that train.