I can’t see them being a giant killer, like InDesign was against Quark. At least, not in book publishing.
The book industry is too heavily invested in Adobe (at least the publishers that I work with). There are still some Quark hold-outs (mainly in the UK), and when the U.S. publisher buys the rights to that book, they have us convert to InDesign.
And that’s what will happen with Affinity Publisher. They will ask that it be converted.
Now–once it comes out, I’m sure the company I work for will be asked if we have it, and I will probably buy a copy for myself to learn it, and the company will buy a couple. But only on the off-chance that we get such a request.
I’ve had the same experience with upgrading my Quarks. We haven’t done a full Quark job since InDesign 2 or InDesign 3. So we’re talking a lot of years. We had a few Quark 7 jobs during that time, but not many.
Anyway–I kept my Quarks upgraded to Quark 8, Quark 9, and recently Quark 10 and 2015. In that time, each and every Quark file had to be converted to InDesign. And Quark 10 and Quark 15–one single job came in. ONE! Talk about a waste of money to upgrade :(
Anyway–I’m rambling on. Right now InDesign is King of the Hill and I can’t see anyone toppling them.