What is the evidence for this? In principle I understand the argument, but in the real world, where have property tax increases in the 5-10% range been definitively linked to home value decreases? Furthermore, what are the severe effects of small decreases in home value?

You don't need any income to take out a reverse mortgage. If you own a $1.5m home, you're a millionaire. Pretending you can't pay Cambridge's very low property taxes is ridiculous.

Let's say that you're a long-time Cambridge resident that owns a house purchased for $150,000 that's now worth $850,000. You aren't rich. Your property taxes are about $2,300/year which helps you "afford to keep living" here.

Let's say that you're a long-time Arlington resident that owns a house purchased for $150,000 that's now worth $650,000. You aren't rich. Your property taxes are $7,300/year - over 3x more than Cambridge.

Cambridge's housing values are high, but not that high that it would put a burden on low-income, long-time residents more than anywhere else. Yes, fixed-income people can face issues with things like property taxes, but arguing that Cambridge should have low property taxes because of some low-income, long-time residents feels like using them to justify low taxes for the majority of rich people.

Why not exempt low-income residents from property taxes if that's the issue? Oh, because rich people want low property tax rates. What about long-time, low-income renters (or any renters)? I guess they can be pushed out.

I do agree that the house-rich might not be able to afford to take out a home equity loan and pay the monthly on that to pay for property taxes. At the same time, Cambridge's property taxes aren't burdensome at all compared to basically anywhere else - even with the high property values. You'd have to find a $200,000 place to get property taxes as low as Cambridge's. So practically everyone in Massachusetts is paying higher property taxes than the Cambridge resident with an $850,000 place.

While a home equity loan might not work, a reverse mortgage might. Reverse mortgages don't require monthly payments and aren't repaid until the borrower moves or dies. If you have an $850,000 place, maybe you can take out a $400,000 reverse mortgage and when you die the bank sells the house and gets their money. You can argue that means their kids might not get to inherit the home, but then why are we concerned about giving some people's kids a windfall while others struggle to make rent?

Likewise, Cambridge could simply allow long-term residents to defer property taxes until a property is sold. Cambridge could even use the city's great credit rating to borrow the money they need today at a very low rate and simply charge the resident that low rate (maybe 1-2%). 20 years later, the resident might have accumulated $50,000 in property taxes and owe $60,000 with the interest and those inheriting the property could decide whether they wanted to pay the taxes and keep the property (getting an $850,000 property for $60k is a steal) or sell the property to pay the taxes (inheriting $790k is pretty sweet too). Plus, given that the owner would have had to pay $50,000 of that $60,000 along the way, that's all money that would be coming out of the kids' inheritance anyway (well, except for the $10,000 in interest). But the point is that Cambridge could just solve the issue for long-term, low-income residents without resorting to lower property tax rates.

The house-rich argument doesn't make sense as an argument to keep Cambridge property taxes low. That doesn't mean that people don't face issues. However, there are much better ways to deal with the issues and there's almost no one in Massachusetts that faces a lower property tax burden than a Cambridge resident in an $850k place - if they're struggling with property taxes in Cambridge, it's not like they'd "be able to stay" in any other town and just got unlucky with Cambridge. Again, that's not to say that people don't struggle, but that it's not a struggle that's unique to Cambridge. Likewise, there are simply better ways of dealing with the issue. Cambridge could simply defer the taxes and use its borrowing power for current spending - and then recoup the taxes later.

Perhaps a reverse mortgage then? If they've been here for decades, they should qualify.