Back when InDesign 1.5 startet, I was working in the graphic-dep. of a local printing company. Quark was "state-of-the-art" and all jobs where done in Quark (or Publisher). Adobe was able to open Quark 4.0 and below documents, as well as Publisher. That was done by reverse engineering (as far as I can remember). If that wouldn't have worked, I doubt that Adobe would have had such a success with InDesign. Publisher was obsolete back than already, but after some months of testing, the whole company switched from Quark to InDesign.
So right now I'm desperatly waiting for news on Affinity Publisher and a beta to test…