The main reason why Javascript is in the discussion is that this is what is used in InDesign and the other Adobe creative apps. Many of the people looking for scripting in Affinity apps will no doubt have built scripts in those and so using Javascript would make sense in that there would be an easier transition in that regard. I havn't used Quark in a long time (v6.5) so I have no idea what that uses other than just Applescript.
There are plenty of Javascript libraries out there, for example https://github.com/exif-js/exif-js - however, that is where the argument for Javascript does fall down in that because Adobe's Javascript implementation hasn't been updated in well over a decade you cannot actually use any Javascript libraries because they are all using newer features that break in Adobe's environment. For example when I built my last large InDesign script I needed something to write tests in - the best I could manage was getting 14 tests running in Jasmine before it would die.
I don't know Python but that's only because I have not yet had a need to learn it. If we get Python-based scripting in Affinity then great, I guess I will have a reason to learn it. I just feel like the focus really needs to be on making sure that whatever implementation is chosen gives us the depth comparable (or better) to that of InDesign as opposed to the half-assed implementation of Acrobat Professional.