The small issue with that answer I'm afraid is does anybody have a crystal ball, who knows now what they want to re-open several years from now? It is much more sensible to either have the means to open .SPP files by it's replacement or a genuinely reliable means to convert them. This may turn out to be an issue for your longer term supportive customers a lot more than the new business you'll get from refreshing your products image and as we all know Serif have depended upon, and given good bonuses to, their existing customers. The "corporate answer" coming out of Serif isn't what many of them will want to hear.
The policy as it stands would rest more upon Microsoft than Serif with regard to "future proofing", I use Capture NX2 for RAW images from my Nikon DSLR which, though it's running happily on W10, was shelved/sold off by Nikon years ago and the new owner used the name within it's Nix Collection and, though Nikon seem to be rolling out "Capture NX-D", it's not a true replacement. So, at some point, upgrades to W10 will make my copy of Capture NX2 useless . . . and the same thing then is destined for presently working editions of PhotoPlus.
If the reason why is genuinely "physical" in that the "engine" within Affinity simply can't communicate then that's the conclusion to your argument but please tell us so . . but if it's based more upon Serif's requirement to earn new business, thus it's a "promotional" decision, then I for one suggest that Serif re-consider.