Sadly, no crossover from PP to Affinity seems likely, so a lot of user support knowledge will evaporate, along with a loyal customer base from the Windows community.
I agree with Bhikkhu Pesala in that Serif should be the people best placed to write a translator for their PP.xml files, but presumably they failed to document its development internally, so are not actually any better placed than anyone else.
Even if Affinity is going to import *.rtf from PPX9, Serif failed to implement much of the last RTF spec in the PP>RTF export anyway. By RFT v1.9 MS had built in to RTF support for footnotes, contents and indexes, its just the PP>export that does not support later versions of the RFT-spec (although some of the information can sometimes be found in hidden characters in a pure text editor, or by a third party *.rtf importer).