This is actually not true. When sallying, if the besiegers failed to take the center during the battle, the walls and gates were unbreached, and the gates were all closed with all the defenders within the walls the battle would end in a draw and the siege would continue. Only if the defender still had units outside the wall or the besieger had an open route to the city center would the autoresolve function take over and decide on casualties.
Many games when I was besieged I'd sally, cause whatever casualties I could with missile troops, then withdraw back into the city and end the battle. Over the course of a couple of turns you could seriously whittle down the besieger that way.
Now, IMO, when they sally and lose, the "armed townsfolk" units should not fully respawn but should remain as they were after the battle. If only 2/60 survive then that's what you should have to fight next turn.
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