History is punctuated by stories of city sieges; from ancient Greece to Sarajevo, the difficulty in taking a dense urban area in which fighters swim as fish in a sea of civilians has been reflected in the consequences contingent on failure or success. A besieging army that takes a city after arduous and dangerous combat with high levels of fatalities traditionally ends it in an orgy of rape, violence, drunkenness and fire, with no quarter for the defenders. A city that withstands a brutal siege works against the besieger in terms of not just a boost to morale for the defenders but a very substantial advantage; Vienna and Stalingrad both changed the course of the enemy invasions. To take high casualties then to withdraw exhausted is as destructive of a failed besieger as a defeat on the battlefield.
So for the Syrian government and rebel fighters, a lot hangs on Aleppo. There are only three possible outcomes. The city falls, and the defenders are slaughtered, or the rebels withstand the assault and Assad is grievously weakened, or the rebels surrender for the sake of a civilian population without food, clean water or medical facilities. Of the three, it is clear that Assad will never lift the siege - he is all too aware of the consequence of failure.
There is therefore only one option for the rebel defenders and their US, EU and Saudi Arabian / Islamist backers - for the sake of humanity, for human dignity, and for peace and mercy, they must surrender the city and secure the best terms that they can.