I could not agree more strongly - nodes are the future of Image editing, and it would bring a real cutting edge to Affinity if they would include the compositor style workflow as an persona; the traditional layer system won't diseappear anytime soon I'm afraid.
perhaps it could be implemented as a "Composite layer"; you could add this placeholder layer into your layer stack, then switch to the Node Compositor Persona and make your nodes edits, which would then be piped back into the layer stack for continued editing if desired.
What make node compositor style editing so powerful is that you can split mulitiple branches off of one image, apply effects separatly and recombine the image later down the line using a multitude mixing/masking methods.
This kind of edit in a traditional layer system would require many copies of the orginal image to start with and a lot of hacking around with groups and masks, which is much less flexible to work with.