When I think megacity I think “arm ants”. They can devour anything! It is easy to face the challenge of a few, but impossible when it is a few thousand.
Even Rambo runs out of bullets and has to sleep. Big cities will be death traps if the grid goes down for 2+ weeks coast to coast.
A hornet’s nest looks very organized and stable, a few bees flying in and out, until you whack it with baseball bat. That is a big city after the grid goes down.