We went through this angst when photo and designer were released (or at least I did). After some deep thought about the reasons offered by the new programming team for the absence of compatibility, I came to the following conclusions;
The programs are not simply mega-updates of the Plus series from Serif - they are totally new programmes with a whole new and very different code base. To make the programmes able to accept Plus files would be nigh on impossible or severely limiting.
To create a conversion programme would entail a lot of work (different code base) - I'd rather the Affinity team concentrated on developing the Affinity series, especially since Publisher is still only in beta.
And with a blinding flash of recognition, I realised that my whimperings about all my 'legacy' Plus files was misplaced. They aren't legacy files because there aren't any new, super-charged Plus programmes that need to accept them. And I don't actually need a conversion programme because the vast majority of my Plus files are complete and I already have an excellent working programme on my PC that can load them and allow me to edit them and resave them. Yes, there are a few projects still in ongoing development and yes, these will either continue to be developed in PagePlus (which won't stop working just because Serif have abandoned it) or if they are likely to have a great deal of longevity, I will have to recreate them in Affinity - but I already have the text, images, icons etc saved on my PC along with the overall design/format so it's not a matter of recreating the content, just dropping it into the Affinity program and doing some reformatting.
Already created projects will stay as PagePlus and new projects will be developed on Publisher
So, for me at least, it's not really an 'end of the World' scenario.
And yes, I am a glass half full sort of guy