Unfortunately, the problem would still remain. The pdf file just doesn't handle transparencies properly - something always gets lost. If anyone has any suggestions for improving that then I would love to hear.
As for being written on the mac, as far as I know it was developed cross platform first and looking at the DLLs (system files) it was probably written in .net core (no obvious java files) so they would very likely have taken the original file format concept or an improvement on it.
Simply renaming the ppx with a .zip extension gives access to the full package but this fails with the afpub. It could just be because they use a different compression/encryption approach rather than an internal change in structure. Looking at a file size comparison however for near identical contents the affinity file is significantly larger for just a grid, a text section and a tiny picture. This might be a way to extract some of the contents to rebuild them in an affinity file but that is such a faff.
Ultimately I have been a loyal serif customer since the first couple of versions, spent hundreds and would like to continue spending hundreds more in future (as would many other of their customers).
Is the refusal not to create an import mechanism down to technological restrictions or the marketing team because they don't want to be associated with the old serif market in an attempt to look like a different company so that the mac users don't consider it an inferior product (although pp9 still has a lot more features than the current publisher version). This would be a terribly sad waste of so much goodwill built up over years. :-(