The main deal with WoS is the fact it is focusing on the established features of the franchise of Ageod titles. Examples include simple supply, diplomacy,attrition, and economics. This is not to water-down the game but rather make a push to release the most bug-free experience for both veteran and new players alike. To add an experimental feature such as breach repairing frankly goes against the main goals for this game in particular.
Also, before a new siege even happens in an Ageod game a week or a month usually passes. This is possibly enough time to have prepared new garrisons and defenses.