Affinity Publisher is much cheaper than Indesign and is available without a subscription. I am convinced that the program will find its buyers.
An open file format would give Serif the chance to open closed InDesign workgroups for afpub users. A practical example: I volunteer my time in a non-profit organization and occasionally design flyers and posters together with a colleague. But she works with InDesign and I use afpub. She saves her work for me as PDF/X-4 files, which I can easily open with afpub. But I have not yet found a satisfactory solution for the opposite direction.
Whether Adobe would then support the format in his program is, of course, on a different page.