The army will wait it out to the last minute (possibly disastrously so as early intervention might be better in cases of large-scale violence) and may be internally divided about how to proceed (hence the hesitation).
Should Morsi be toppled, it will create an enormous problem with the Muslim Brotherhood and other Islamists for years to come. They will feel cheated of legitimately gained power and Egyptian politics will only grow more divisive and violent.
Whatever alliance came together behind the Tamarrod protests will fall apart the day after its successful, because its components are as incompatible as the alliance that toppled Hosni Mubarak.
The leadership around the NSF (ElBaradei, Moussa, Sabahi etc.) has followed rather than led Tamarrod and will not be able to provide effective leadership in the coming days. Only the army can.
If Morsi remains and the protests are repressed or simply die out, the country will nonetheless remain as difficult to govern considering Morsi's lack of engagement with the opposition.