You're right, and Prop 13 doesn't get nearly enough hate in these discussions. It's not just that people are sitting on investments like domain campers, though - it makes it impossible for many people to move, because if they did move to an equivalently valued home, they'd be paying many times their current property tax. This restricts the liquidity of the housing market and encourages hoarding, which leads to skyrocketing prices among the few houses that do go on the market.
If that was slowly phased out, prices would come down as more houses came on the market, and the houses that those people wouldn't represent such a huge jump in ongoing property taxes.
How does repealing Prop 13 change anything? Wouldn't everyone just end up paying the same or more taxes? Looking at property tax rates in other states (which presumably don't have such a law), we're about on par with them.
The people who can't move because of increased taxes would be forced to flee the state maybe?
We may be on par with them in terms of nominal rates, but those other states' property taxes rise with the value of the houses every year, whereas CA prop tax amounts are based on the cost at the time of purchase and adjusted at a max of 2%/year, even if their house appreciates by 10%, as it does some years. People who bought their houses in 1975 are paying taxes based on what their houses were worth in 1975 times roughly 1.02^40, or 2.2x. In the same period, the median home price went from $41,600 to $478,700, a rise of almost 12x. So, if they moved to an equivalently valuable home, they'd pay about 5.5x the amount of tax going forward, which can be very significant.
Housing prices take into account the amount of taxes one expects to have to pay, so if they're no longer locked, and one expects prices to rise, then the price one will be willing to pay will be lower. Also, prices will fall as more supply opens up, since there won't be such a big advantage to sitting on houses anymore, and people are more free to move to less expensive areas. Currently there's a bit of a rent control situation where the low rates can be passed on to your descendants, and there's a big incentive to stay in it rather than selling it and moving farther out, even if the place farther out is half the price. This effect is more pronounced on the high demand areas where prices have risen more.
Over time, the prices would adjust and settle. People may end up paying more tax overall, but we can play with the rate, or we could decide that the schools could use more funding (in many places, they could), and that we should keep the rate where it is. As it is now, the people who are just moving to CA are heavily subsidizing people who have lived here for a long time, and the lack of liquid housing supply is causing prices to shoot up.
"... but those other states' property taxes rise with the value of the houses every year, ..."
In Michigan, it's a bit more complex. The assessor calculates a State Equalized Value (SEV) on which the tax is based, and that cannot rise faster than the rate of inflation, or 5%, whichever is less. I assume lots of states have odd property tax quirks.
People who already own property in places where values are rising have no incentive to do anything to counteract the rising trend. So we have the spectacle of Mountain View denying Google's request for approval to build apartments for their employees near their campus -- at a time when housing in Silicon Valley is getting absurdly expensive.
I explained it earlier. Repealing prop 13 forces people holding on to undeveloped land to sell it to someone who can make use of it.
It's funny that people accept taxes of up to 40% on wages (things the middle class earns), but can't understand how taxing vast landed estates (things the rich owns) would "help." Suggest thinking this through more clearly.
If that was slowly phased out, prices would come down as more houses came on the market, and the houses that those people wouldn't represent such a huge jump in ongoing property taxes.