The contract: If you save, invest, and pay off your home mortgage so that you own it free and clear, later in life when you are retired and living on a fixed income no one can come along and confiscate the property that you bought and paid for fair and square.
The contract breech: Property taxes going up so fast that the only possible solution was to sell your home. Now, of course the value of the home was going up, which was what was driving the taxes. But forcing pensioners to sell their homes, disrupting/destroying a life-time of plans and work was never going to be a smooth ride -- who could have thought that it would be?
The government pre-Prop 13 was viewed as kleptocratic by a sizeable portion of the population, with arguably good reason.
So, do you have a better solution that has empathy for pensioners that played by the rules all their lives and have plans to settle into modest enjoyment of their so-called "golden years"? Prop 13 was an attempt to stop perceived thievery, a crude bludgeon, perhaps, but a sufficient number of desperate people saw it as a solution to get it enacted.