DUS is AB's main hub and comparably central and close to BRU for intra-EU flights whilst avoiding UK taxes for long haul flights. FRA, ZRH, and CDG are actually adding a lot of distance for US to EU connections since they're further south on GC routes.

I'd say OW, by which I mean BA, doesn't value AB very highly since they were traditionally a LCC ,competitor, and still partially owned by another competitor so they are intentionally leaving them underused for long-haul whilst letting them have some intra-EU flights.

If they do leave, it will purely be because Etihad is the largest shareholder. I doubt it would be a wise business decision, but staying in also means playing second fiddle to BA trying to route everything through London even if it is a hodgepodge of disconnected airports and failing ATC computers.