To the extend that I understand what's happening in RAW conversions, I think there is a great deal more magic and engineering in it than an open source set of libraries can competitively handle. Adobe and Phase One have spent years getting their algorithms to the points they are now.
Depending on what you would call the "top" non-proprietary RAW converters (perhaps ACR, Apple, C1, and Iridient?), you'll find adherents to each. And the careful studies I've seen suggest that each can be "better" on different images, but one doesn't dominate across all images. ACR has taken many hits for its handling of X-Trans files, for example.
But Affinity does not yet seem to be in the upper echelon of RAW converters.
My point is perhaps they don't have to be, if they can use/license another converter. Apple's seems most logical, as there would be little to keep Apple from allowing it - perhaps for free. But can Apple keep up with the others for top quality work? Abandoning Aperture doesn't leave me with much trust.
If Serif Labs is going to develop the RAW conversion expertise to go head-to-head with the "big boys," they haven't talked much about it, yet.