It still seems a bit difficult to find a replacement for these two on macOS and Linux. Intuitive interfaces focusing on primary image editing functions and stable output to various formats are probably what is missing for non-professional users like me.
GIMP seems like a great project but has a terrible overblown UI. I tried Photoshop / Illustrator for a year as you get the CS4-subscription at a low price from my university. A few features, e.g. how embedded images were handled are nice, but the software is massive and the interface is so weirdly complex that I had to search for guides for the simplest tasks (who is profiting from having to "study" CS4...?). With PSP7, which is decently powerful I went through a couple of tutorials and was set up for years. Also, I had spent half an hour to deactivate all the spyware that comes with CS4, this is a no-go.
You can read that I am just really ignorant about modern graphics editing workflows. I'm not a graphics professional, but need to make "production level" graphics every few months. I need relatively technical vector illustrations (e.g. of experiment flows, NN models) and clean figures of scientific results (raster) in a visualization-heavy field.
For vector graphics I now really like Inkscape with its SVG basis. I still go back to Dia a lot though whenever the illustration fits their limited (but sufficient) scope. Both can be used on Linux and macOS.
Still searching for a powerful but plain & simple (and stable!) raster graphics editor though. Currently Krita looks nice, but it is clearly in development stage. Would pay for either Acorn or Pixelmator if I wasn't concerned that they will also start emulating the UI of Photoshop in the future.