If the Nix were actually ever in a position to win the Australia Cup, the premiership or the toilet seat, I think our "assured position" in the comp would suddenly evaporate.
Not sure I can agree. If we win the pre-season Australia Cup, other teams would probably politely congratulate us, especially if we beat other A-League team in the final. That Cup is simply a warm up stretch for A-League, and a sop thrown to the non A league teams that cannot hope to be promoted.
And if we win the premiership, it would be a sign of us having a great coach and a good team etc etc. Possibly winning the GF, especially if we hosted it, might be a source of rumblings as it would be seen as a commercial problem, but the final has been sold to Sydney for several seasons ahead already, so commercially the league will not miss out. This, applied to last GF, meant that Melbourne City played away from home.
Either way, we would not be allowed to compete in the AFC comp even after winning the premiership or the GF, so it would be a sentimental symbolic achievement, and the third runner up team would say "gee, thanks Nix".