It's not obvious to Matt Levine that AIG in 2015 is actually politically too big to fail, or that investors think it is.
Carl Icahn's letter to AIG telling it to break up includes this argument: "Because of AIG's size and interconnectedness" the Financial Stability Oversight Council ("FSOC") has deemed AIG a non-bank SIFI, subjecting the company to Federal Reserve oversight and increased capital requirements.
The regulators have made clear that the best outcome is for SIFI's to shrink and "reduce their systemic footprint." If nothing is done, returns and AIG's competitive position will continue to suffer as the SIFI regulation, including its costs and capital requirements, is fully implemented.
One thing you can say about, for instance, calls to break up JPMorgan, is that the very real costs of SIFI (systemically important financial institution) regulation are probably offset by the funding advantages of being a SIFI. Like, if you are a too-big-to-fail bank, you'll have to deal with expensive regulation, but being too big to fail also means you can borrow cheaply.
That is kind of the deal. It seems pretty true of JPMorgan. It is less obvious to me that AIG, in 2015, is actually — politically — too big to fail, or that investors think it is, or that it has that funding advantage.