You have mis-stated the situation. It is absolutely not true that "Many homes in California are not maintained and updated for this very reason [reassessment]"

You have mis-stated the situation. It is absolutely not true that "Many homes in California are not maintained and updated for this very reason [reassessment]". Only the cost of the improvement, such as an addition to the building, is added to the existing assessed value. The entire property is not reassessed. And if the update is just a remodel (such as new flooring and tile in your bathroom, or new cabinets/countertops in your kitchen), there is no reassessement. Example: a house was purchased 30 years ago for $200K, so that is the base year value (goes up 2% per year per Prop 13). If you build an additional bedroom onto the house for $50K, then only $50K is added to your assessed value.