Even if the cost base were the same, it's only half of the equation, the other is the network effect on revenues.

Currently, QF's SIN-LHR and SIN-FRA flights get passenger feed from their SYD-SIN, MEL-SIN, BNE-SIN, ADL-SIN and PER-SIN flights. However, SQ can fill their SIN-LHR and SIN-FRA flights with connecting passengers from their Australian flights, but also from their flights coming into Singapore from other regional destinations.

If QF were to start say SIN-FCO, it would have to rely on it's Australian flights and local Singapore sales to fill those seats because it doesn't have any other network at SIN. Singapore Airlines can fill their SIN-FCO flight with passengers connecting from Manila, Jakarta, Auckland, Hanoi etc.

It was part of the reason QF were trying to start an airline based out of SIN (RedQ), why they should have partnered up with CX if they actually could get along, and why they have now gone with EK.