The idea is that, even though additional capital surcharges above the (now higher) standard Basel III minima make it less likely that a G-SIB could fail, they do not make it impossible. Such entities are sufficiently large and interconnected that an uncontrolled failure could easily cause systemic disruption.
Therefore, it is argued that further loss-absorbing capacity is needed, to be called on at the point of non-viability, so as to allow vital functions to continue and non-critical operations to be wound down in a controlled way. This limits adverse spillovers to the system and the economy.